Preheat the oven to 350°F. Line a 9×13 baking dish with parchment paper, and set it aside.
In a medium-sized bowl, stir together the flour, baking powder, and salt, and set aside.
Place the melted butter into a large bowl. Add the brown sugar, mashed banana, egg, and vanilla, and whisk everything until smooth and combined.
Add the flour mixture to the bowl with the wet ingredients and stir until combined with no dry patches.
Spread the batter evenly into the prepared baking dish. Bake for 30-35 minutes until the edges are golden brown and a toothpick inserted into the center comes out clean. A few crumbs left on the toothpick are okay, but if there is any wet batter, it needs to bake longer.
Place the baked blondies on a wire rack to cool completely.
While the blondies are cooling, make the frosting. Place the cream cheese and butter into a large bowl, and whip it until smooth with an electric hand mixer. You can also do this in the body of a stand mixer with the paddle attachment.
Add the powdered sugar a little at a time until combined, and scrape down the sides as needed. Add the vanilla, then turn the mixer to medium speed, and whip it for 3 minutes, this will make the frosting light and fluffy.
Lift the blondies out of the baking dish using the parchment paper to help. Smooth the cream cheese frosting on top and cut it into bars, serve immediately.
Notes
Use recipe bananas like you would for banana bread.
This can be frozen, see above for how to do that.
You can add other additions or toppings to this recipe, see some of my ideas above.
This can easily be reduced by half, just make half the recipe and spread the batter into an 8×8 baking dish, baking times may vary..
These must be stored in the fridge since they are frosted with cream cheese.
1 large or 2 medium bananas should be enough for 3/4 cup mashed banana.