Preheat oven to 350°F. Line a 9x13” pan with foil and spray with cooking spray.
Cream brown sugar, peanut butter, and butter in the bowl of an electric mixer fitted with a paddle attachment. Add eggs and vanilla and beat until combined. Add baking powder, salt, and flour and mix.
Stir in 2 cups of the chopped Butterfingers.
Press into prepared pan (dough is sticky, spray your hands with cooking spray to avoid it sticking to your hands).
Sprinkle the remaining 1 cup of chopped Butterfingers over the top and press lightly to make them stick to the surface.
Bake for about 22-25 minutes. (Be careful not to overcook - the center will still be jiggly when you take these out of the oven and they will finish cooking and firming up as they cool.) Cool completely before slicing into squares.