Preheat the oven to 375 degrees and line an 8x8 pan with parchment paper to prevent sticking.
Place the butter in a saucepan and melt the butter. Cook until the butter solids are browned and smell nutty. This will take about 10 minutes on medium heat. Stay close and stir the butter constantly. Pour into a separate bowl and let that cool.
While the butter cools, mix the flour, baking powder, and salt in a small bowl. Set that aside.
In the bowl of a stand mixer fitted with the paddle attachment, add the brown sugar, granulated sugar, and brown butter. Beat on medium speed for 2-3 minutes. Scrap the bowl and continue mixing. Add the eggs, one at a time, mixing between each egg.
Add the vanilla and continue mixing. Be sure to scrape the bowl regularly while you mix.
Add the dry ingredients to the mixer with the mixer on low. Mix until the dry ingredients are combined.
Add the M&Ms and chocolate chips and mix until everything is just combined.
Pour the batter into the prepared 8x8 pan and top with some more candy if you would like. Bake for 35-40 minutes until the edges are just browned. Take the bars out of the oven and allow them to cool before cutting. Enjoy!