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Monster cookie bars are everything you love about a classic monster cookie packed into one thick, chewy, no-roll bar. They are loaded with peanut butter, oats, melty chocolate chips, peanut butter chips, and a rainbow of M&M’s, so every bite is a little party. I make a pan whenever we need a guaranteed crowd-pleaser, and they disappear even faster than our chocolate covered peanut butter pretzels.
Skip scooping dozens of cookies and make one easy pan of these thick, chewy bars instead.
Monster Cookie Bars Quick Look
- 🕒 Prep Time: 10 minutes
- 🌡️ Cook Time: 30 minutes
- ⏳ Total Time: 40 minutes
- 🍽️ Serving: 15 bars
- ⚡ Calories: 410kcal
- 🌶️ Flavor Profile: Rich peanut butter and chocolate with chewy oats and candy crunch
- ✋ Difficulty: Easy, no chilling or rolling, simpler than our chocolate chip bundt cake
Quick Answer
Stir together the dry ingredients of flour, oats, baking powder, and salt. Cream the butter and peanut butter, beat in the granulated and brown sugar, then add the eggs and vanilla. Mix in the dry ingredients, fold in the M&Ms, chocolate chips, and peanut butter chips, and press the dough into a baking dish. Bake at 350 degrees for 25 to 30 minutes until the center is just set, then cool completely before slicing. Pressing the dough into a pan instead of scooping cookies is what gives you thick, chewy bars every time.
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Why This Recipe Works
Click to see the technique science
- Bars are easier than individual cookies. You press all the dough into one pan, so there is no scooping, rolling, or batches of baking sheets.
- Peanut butter keeps them soft and chewy. Creamy peanut butter adds richness and moisture so the bars stay tender, not crumbly.
- Oats add the signature monster texture. Old-fashioned rolled oats give that classic hearty, chewy bite monster cookies are famous for.
- Three mix-ins mean maximum flavor. M&Ms, chocolate chips, and peanut butter chips load every bite with color, crunch, and melty chocolate.
- Underbaking keeps them gooey. Pulling them when the center is just set leaves the middle soft and chewy instead of dry.
- They feed a crowd. One pan slices into 15 generous bars, perfect for parties, bake sales, and lunchboxes.
Why You’ll Love This Recipe
- They give you all the monster cookie flavor with none of the scooping or rolling.
- They are a fun, colorful treat that travels well, great for bake sales like our baked chocolate chip cake donuts.
- The combination of peanut butter, oats, chocolate, and M&Ms is impossible to resist.
Key Ingredients
These thick monster cookie bars come together with classic baking staples plus three fun mix-ins.
- Peanut butter: Creamy peanut butter is the heart of the dough, adding richness and that classic monster cookie flavor.
- Old-fashioned oats: Rolled oats give the bars their signature hearty, chewy texture. Do not swap in instant oats.
- Granulated and brown sugar: Together they sweeten the bars and keep them soft, with brown sugar adding a chewy, caramel note.
- M&Ms: The colorful candy that makes these bars look like a party and adds a fun crunch.
- Chocolate and peanut butter chips: Two kinds of chips melt into the dough for double the chocolatey, peanut buttery goodness.
See recipe card for exact quantities.
Variations and Substitutions
These monster cookie bars are easy to customize to what you have on hand.
- Make them gluten free: Use a one-to-one gluten free flour blend and certified gluten free oats.
- Switch the candy: Swap the M&Ms for Reeses Pieces, chopped peanut butter cups, or seasonal colored candies.
- Add nuts: Fold in a half cup of chopped peanuts for extra crunch and nutty flavor.
- Go all chocolate: Use all chocolate chips and skip the peanut butter chips for a more classic chocolate chip bar.
- More easy treats: Loving the bar life? Try our mini cheesecakes or chocolate covered peanut butter pretzels next.
How to Make Monster Cookie Bars
- In a medium bowl, stir together the flour, oats, baking powder, and salt, then set it aside.
- Cream the softened butter and peanut butter until smooth, then add the granulated and brown sugar and beat for 3 minutes until fluffy.
- Add the eggs one at a time, mixing fully after each, then stir in the vanilla.
- Add the dry mixture to the wet ingredients a little at a time until fully combined.
- Stir in the M&Ms, chocolate chips, and peanut butter chips until they are evenly distributed throughout the dough.
- Press the dough into a prepared baking dish, bake at 350 degrees for 25 to 30 minutes until the center is just set, then cool completely before cutting into bars. Enjoy!
Recipe Tips & Tricks
- Do not overbake. Pull the bars when the center looks just set. They firm up as they cool and stay chewy instead of dry.
- Use old-fashioned oats. Rolled oats give the right chewy texture. Quick or instant oats turn the bars mushy.
- Line your pan. A parchment sling makes it easy to lift the whole slab out for clean, even bars.
- Press the dough evenly. An even layer bakes uniformly, so dampen your fingers or use a spatula to smooth it out.
- Save some candy for the top. Press a few extra M&Ms onto the surface before baking for a bakery-style look.
- Cool completely before slicing. Warm bars fall apart, so let them set up fully for clean squares.
Serving Ideas and Suggestions
These monster cookie bars are perfect just as they are with a tall glass of cold milk. For a fun dessert board, cut them into smaller squares and serve alongside our mini cheesecakes and chocolate covered peanut butter pretzels.
Warm a bar for a few seconds in the microwave and top it with a scoop of vanilla ice cream and a drizzle of caramel sauce for an easy a la mode dessert that feels extra special.
They are also fantastic for sharing. Pack them in lunchboxes, bring them to bake sales, or add them to a holiday cookie platter with our chocolate chip bundt cake and a layered trifle for a dessert spread everyone will love.
Monster Cookie Bars FAQs
Two things. Pressing the dough into a pan creates a thick layer, and pulling the bars from the oven when the center is just set keeps the middle soft. The peanut butter and brown sugar also add moisture, so the bars stay chewy for days when stored in an airtight container.
Yes. Use a quality one-to-one gluten free flour blend in place of the all-purpose flour and make sure your oats are certified gluten free. The texture stays thick and chewy, and most people will not be able to tell the difference.
Not at all. The two kinds of chips add great flavor, but you can use all chocolate chips, all peanut butter chips, or swap in butterscotch or white chocolate chips. Just keep the total amount about the same so the dough holds together.
Store the cooled bars in an airtight container at room temperature for up to five days. Layer them between sheets of parchment so they do not stick together. They stay soft and chewy thanks to the peanut butter and brown sugar in the dough.
Absolutely. Wrap the cooled bars individually or layer them in a freezer-safe container with parchment between the layers and freeze for up to three months. Thaw at room temperature, or warm a bar in the microwave for a few seconds for a fresh-baked taste.
A 9 by 13 inch baking dish works well for a full batch of 15 bars. For thicker bars, use a 9 by 9 inch pan and add a few minutes to the bake time, watching closely so the center sets without overbaking.
In the mood for more peanut butter and chocolate? Make our chocolate covered peanut butter pretzels next.
Love a fun, colorful cookie? Our pop tart sugar cookies are loaded with crushed Pop-Tarts and sprinkles.
Cookie tray planning? Add these fudgy cosmic brownie cookies with rainbow candy chips.
Love an easy bar? These oatmeal chocolate chip cookie bars bake in one pan.
Monster Cookie Bars
Ingredients
- 1 ¼ cups all-purpose flour
- 1 cup old-fashioned rolled oats
- 1 teaspoon baking powder
- ½ teaspoon fine sea salt
- ½ cup unsalted butter softened
- ½ cup creamy peanut butter
- ¾ cup granulated sugar
- ¾ cup light brown sugar packed
- 2 large eggs
- 1 teaspoon vanilla extract
- 1 ½ cups M&M’s
- ¾ cup semi-sweet chocolate chips
- ¾ cup peanut butter chips
Instructions
- In a medium-sized bowl stir together the flour, oats, baking powder, and salt, set aside.1 ¼ cups all-purpose flour, 1 cup old-fashioned rolled oats, 1 teaspoon baking powder, ½ teaspoon fine sea salt
- In the body of a stand mixer with the paddle attachment cream together the butter and peanut butter until smooth.½ cup unsalted butter, ½ cup creamy peanut butter
- Add the granulated sugar and brown sugar, stir it in until combined then place the mixer on medium-high speed and cream together for 3 minutes, scrape down the sides as needed.¾ cup granulated sugar, ¾ cup light brown sugar
- Add the eggs in one at a time, fully mixing in the first before adding the next. Stir in the vanilla.2 large eggs, 1 teaspoon vanilla extract
- Add the dry mixture to the wet a little at a time until fully mixed in.
- Stir in the M&M’s, chocolate chips, and peanut butter chips until incorporated throughout the dough.1 ½ cups M&M’s, ¾ cup semi-sweet chocolate chips, ¾ cup peanut butter chips
- Press the cookie dough into the prepared baking dish. Bake for 25-30 minutes until a toothpick inserted into the center comes out mostly clean. Do not overbake.
- Let cool completely before cutting into bars.
Notes
- Line your pan: Use parchment paper or foil with extra edges hanging over the long sides—makes lifting the entire block of bars out a breeze.
- Use a large mixing bowl: It’s easier to fold in all the goodies without spillage, especially if it’s your first time.
- Don’t Overbake: Remove them when the top turns golden brown but the center may still look a tad soft. They set more as they cool, preventing dryness.
- Weigh your flour: If you have a kitchen scale, it can help avoid over- or under-measuring. This is a small step that goes a long way for the best texture.
- Go big or small: If you want thick bars, use a smaller pan. For thinner bars and less bake time, use a larger pan like a 9×13.
- Test with a toothpick: Or test the center for a bit of crumb, not raw dough. That’s how you get the best results for a chewy cookie bar that’s not overdone.
Nutrition
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