Preheat your 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 cocoa powder, granulated sugar, brown sugar, and melted butter until smooth.
Add the eggs, one at a time, whisking well after each addition to incorporate fully. Then, stir in the vanilla extract.
Gently fold in the flour and salt, mixing until just combined. Be careful not to overmix; some lumps are okay.
Stir in the peanut butter until fully incorporated. It's okay if the mixture is a bit thick.
Pour half of the brownie batter into the prepared pan. Gently spread it out evenly using a spatula.
Sprinkle the halved mini peanut butter cups evenly over the batter, and if desired, sprinkle chocolate chips on top.
Carefully spread the remaining brownie mixture over the peanut butter cups, smoothing it out.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to keep them fudgy.
Once baked, remove from the oven and allow to cool in the pan for about 10-15 minutes. Then, transfer to a wire rack to cool completely before cutting into squares.
Notes
Serve warm with melted peanut butter or chocolate sauce, or with vanilla ice cream.