Preheat oven to 350°F. Line or grease an 8x8 inch baking pan. Stir 3/4 cup all-purpose flour, 1/2 cup cocoa powder, 1/2 teaspoon baking powder, and 1/2 teaspoon salt together in a bowl.
In a medium sauce pot over medium heat, melt 1/2 cup butter. Continue to cook while it foams and stir constantly until it browns and you see some brown specks at the bottom and it starts to smell a bit toasted (i.e. heaven). Remove from heat. Once butter cools a bit, add 1 1/2 cups white sugar and 2 teaspoons vanilla extract. Next, beat in the 3 eggs one at a time with a spoon. Gradually add the flour mixture, and stir the batter until it's evenly moistened.
Cut 9 fun sized Butterfingers into thirds.
Add your Butterfinger pieces and 1 cup mini Peanut Butter Cups cups to the batter and stir to combine.
Spread the batter into the bottom of the prepared baking pan.
Bake in the preheated oven until the brownies begin to pull away from the sides of the pan, and a toothpick inserted into the center comes out clean (except for melted chocolate), 35-40 minutes. Cool in the pan, cut into squares, and serve.