Preheat your oven to 350°F (175°C). Line an 8×8-inch pan with parchment, leaving a slight overhang to lift the brookies out cleanly.

In a medium bowl, whisk melted butter and granulated sugar until glossy. Stir in eggs and vanilla until smooth—no lumps in sight.

Sprinkle flour, cocoa powder, baking powder, and salt over the wet mix. Gently fold with a spatula until just combined; avoid overmixing or the brownies turn cakey.

Spoon half of the chocolate mixture into the prepared pan, smoothing it with the back of your spoon to the edges.

In a small bowl, stir peanut butter with powdered sugar (if using) until creamy. Dollop evenly over the chocolate layer.

Drop remaining brownie batter over peanut butter in spoonfuls. Use a butter knife or skewer to swirl the two layers together—think marbled art, not muddy brown.

Scatter chopped peanuts or peanut butter chips on top for texture.

Bake 20–24 minutes, until the edges pull away slightly and a toothpick shows a few moist crumbs. Don’t overbake—these bars set as they cool.

Let the brookies cool in the pan for 15 minutes, then transfer to the fridge for at least 30 minutes to firm up. This step makes slicing neat bars a breeze.

Lift parchment out, slice into 12–16 squares with a sharp knife (wipe blade between cuts), and dig in.