Preheat and Prepare. Preheat your oven to 350°F. Grease and flour an 8x8-inch baking pan.
Combine Dry Ingredients. In a mixing bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
Melt Butter and Peanut Butter. In a microwave-safe bowl, melt the butter and peanut butter together in 20-30 second intervals, stirring until smooth.
Mix Wet Ingredients. Add the sugar, eggs, and vanilla extract to the melted butter and peanut butter mixture. Stir until well combined.
Combine Wet and Dry Mixtures. Gradually add the dry ingredient mixture to the wet ingredients. Mix until you have a smooth brownie batter.
Create the Swirl. Pour the brownie batter into the prepared pan. Drop spoonfuls of peanut butter onto the batter. Use a knife to create a marbled effect by swirling the peanut butter into the batter.
Bake. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.
Cool and Enjoy. Allow the brownies to cool in the pan for about 10 minutes, then transfer them to a wire rack to cool completely. Once cooled, cut into squares and savor the nutty, chocolaty goodness.