Preheat your oven to 350°F (175°C).
Grease a 9x13 inch baking dish or line it with parchment paper.
In a large mixing bowl, combine melted unsalted butter with granulated sugar and brown sugar until well-blended.
Add in the eggs one at a time along with the vanilla extract, mixing well after each addition.
In another bowl, whisk together all-purpose flour, cocoa powder, salt, and baking powder until combined. Gradually fold this dry mixture into your wet ingredients until just combined; do not overmix.
Gently fold in melted creamy peanut butter until fully incorporated into the batter followed by the colorful Reese's Pieces candies. Pour this mixture into your prepared baking dish and spread it evenly before baking.