Preheat the oven to 350°F. Line an 8x8-inch pan with foil or parchment paper, leaving an overhang, and spray with nonstick cooking spray.
In a small saucepan over medium heat, melt the butter and brown sugar, stirring constantly, until smooth. Remove from the heat, and stir in the peanut butter. Allow to cool while combining the flour, baking powder, baking soda and salt in a large mixing bowl.
Add the egg and vanilla extract to the peanut butter mixture and stir to combine. Pour the mixture into the prepared dry ingredients, and stir until well combined. The batter will be thick. Fold in half of the peanut butter chips, peanut butter cups, and chocolate chips, reserving the rest for sprinkling on top.
Transfer the batter to the prepared pan, spreading evenly to the edges of the pan. Sprinkle with the remaining peanut butter chips, peanut butter cups, and chocolate chips, lightly pressing them into the batter.
Bake for 20 minutes, or until the top is golden and the edges are slightly browned. Do not overcook. Allow to cool in the pan. Cut into 9 squares and serve.
Notes
This recipe can be easily doubled to fit a 9x13-inch pan.