Preheat oven to 350 degrees.
Melt butter in microwave in microwave safe dish (30ish seconds) and let cool on counter.
Add the eggs and vanilla to a large bowl and beat eggs with hand mixer. Add the brown sugar to the mix. Combine well. Add butter and mix. In separate bowl, mix the remaining ingredients except for the chocolate additions.
Mix dry ingredients together and add a little at a time to the wet mixture. Once wet and dry mixtures are combined thoroughly, fold in the cadbury chocolate pieces and chocolate chips with a spoon or spatula.
Pour mixture into a greased 13×9 pan.
Place dollops of the nutella around the pan and in a figure 8 motion swirl it into the mixture with a skewer or butter knife. This will be a little difficult due to the candy pieces but just work around them.
Place in oven and bake for 20-25 minutes or until a toothpick inserted into center comes out clean. Let cool on a wire rack.
Once cool, cut into bars and enjoy!