Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Whisk melted butter, granulated sugar, and brown sugar in a large bowl until glossy.
Beat in eggs and vanilla until the mixture looks thick and smooth.
Sift in flour, cocoa, baking powder, and salt. Fold just until no dry streaks remain.
Stir in chocolate chips, stopping as soon as they’re evenly dispersed.
Spread batter into the prepared pan, smoothing the top with a spatula.
Warm peanut butter until pourable, then dollop over batter and swirl with a knife.
Bake 24–28 minutes, until the center is set and a toothpick comes out with moist crumbs.
Cool completely in the pan, lift out using parchment, and slice into squares.