Preheat oven to 350°F. Spray a 8×8 pan with nonstick cooking spray.
In a large mixing bowl, whisk together melted butter and brown sugar.
Add vanilla extract and egg; whisk until combined.
Add flour, baking powder, baking soda, and salt. Stir until combined. Do not over-mix. Fold in Reese’s Peanut Butter Cup Minis and Reese’s Pieces. Spread evenly into the prepared pan.
Bake for 25-27 minutes or until a toothpick inserted in the center comes out clean.