Preheat the oven to 350°F (175°C) and line a baking pan with parchment paper. In a large bowl, whisk together melted butter, granulated sugar, and brown sugar until smooth. Add the eggs and vanilla extract, mixing until fully incorporated.
In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder. Gradually fold the dry ingredients into the wet mixture, stirring gently to combine. Be careful not to overmix.
Gently fold in the chopped peanut butter cups, ensuring they are evenly distributed throughout the batter. The creamy peanut butter filling will melt slightly, adding extra richness.
Pour the batter into the prepared pan and spread it evenly. Bake for 30 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. Let the brownies cool in the pan before slicing.