Preheat oven to 350 degrees. Line a square 9x9" baking dish with a parchment paper sling or spray with baking spray.

In a large microwave-safe bowl, combine the bittersweet chocolate and butter. Microwave on high in 20-30 second bursts, stirring well between bursts of heat until melted and smooth. Whisk in cocoa powder.

Add granulated sugar and brown sugar, whisking well to combine. Add eggs and vanilla, beating well either by hand with a whisk or using an electric mixer.

Sprinkle the flour and salt over the brownie batter and gently stir it in with a wooden spoon or spatula. Stir in most of the chopped peanut butter cups.

In a separate bowl, make the peanut butter swirl by beating the melted butter, peanut butter, powdered sugar, and vanilla together until smooth.

Drop about ⅔ of the brownie batter into the prepared pan by spoonfuls. Fill in gaps with spoonfuls of the peanut butter mixture. Drop remaining brownie batter on top, then swirl both together with a knife.

Bake for 30 to 35 minutes, until a toothpick inserted into the center of the brownies comes out mostly clean with only a few crumbs attached. Be sure not to overbake.

Cool completely before cutting. Store in an airtight container for up to 5 days on the counter or in the fridge.