Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, combine the melted butter, granulated sugar, and light brown sugar. Mix until well combined.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and both types of peanut butter until smooth.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the peanut butter mixture, stirring until just combined.
Fold in 1 cup of REESE’S Pieces gently into the batter. Pour the brownie batter into the prepared baking pan and spread it evenly.
Sprinkle additional REESE’S Pieces on top of the batter for decoration. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 15 minutes before cutting into squares. For best results, chill the brownies in the refrigerator for an hour before serving to enhance the texture.