Preheat the 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, whisk together the melted butter, granulated sugar, and brown sugar until smooth.
Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.
In another bowl, sift together the flour, cocoa powder, and salt. Gradually fold the dry ingredients into the wet ingredients, mixing until just combined.
Gently fold in the creamy peanut butter until fully incorporated. Take care not to overmix.
Add the chopped mini peanut butter cups and chocolate chips to the batter, and fold them in carefully.
Pour the brownie batter into the prepared baking pan, spreading it evenly.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (but not wet batter).
Allow the brownies to cool in the pan on a wire rack for at least 15 minutes before slicing into squares.
Notes
Serve the brownies on a platter sprinkled with extra mini peanut butter cups on top. Drizzle with melted chocolate and a sprinkle of sea salt for an added touch!