Preheat oven to 350 degrees. Line 8-inch square baking pan with foil (letting foil hang over sides) and grease.

Melt butter and chocolate together in the microwave, stirring often, 1-3 minutes. Let cool slightly.

In a medium bowl, stir together flour, baking powder, and salt.

In a large bowl, whisk together sugar, eggs, and vanilla. Whisk in the melted chocolate mixture until combined. Stir in the flour mixture until just incorporated.

Scrape batter into the prepared pan and smooth the top.

Drop the peanut butter in small dollops over the brownie batter, then run a knife through the batter to swirl.

Bake brownies 22-27 minutes, rotating pan halfway through, until a toothpick inserted comes out with a few moist crumbs.

Let brownies cool completely in the pan on a wire rack, for about 2 hours. Remove brownies from pan by grabbing two sides of foil and lifting. Cut into squares.