Heat the oven to 350°F (175°C). Line an 8- or 9-inch square metal pan with parchment paper, leaving an overhang on two sides, and lightly grease the parchment.

Whisk the melted butter and granulated sugar together in a large bowl. Add the room-temperature eggs one at a time, whisking well after each addition, then mix in the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, and fine salt until evenly combined and free of visible cocoa lumps.

Fold the dry mixture into the butter mixture just until no dry streaks remain. Do not overmix, since excess stirring can make the brownies less tender.

Fold about two-thirds of the chopped miniature peanut butter cups into the batter. Spread the thick batter evenly in the prepared pan.

Drop small spoonfuls of the optional smooth peanut butter over the batter and gently swirl it with a knife. Scatter the remaining chopped peanut butter cups over the top.

Bake for 28 to 35 minutes in an 8- or 9-inch square pan, beginning to check at 28 minutes. The edges should be set, while a toothpick inserted in the center should show moist, fudgy crumbs but no wet batter.

Cool the brownies completely in the pan for at least 1 hour. For the cleanest, fudgiest cuts, refrigerate them for 30 to 60 minutes, lift them out using the parchment, and slice.