Preheat your oven to 350°F (175°C). In a medium saucepan, melt the unsalted butter over low heat. Remove from heat and stir in the granulated sugar, eggs, and vanilla extract until well combined.

In a separate bowl, whisk together the cocoa powder, flour, salt, and baking powder. Gradually add this mixture to the butter mixture, stirring until just combined and smooth.

In a small microwave-safe bowl, warm the creamy peanut butter in the microwave for about 20-30 seconds until it is slightly melted and easy to swirl.

Pour the brownie batter into a greased 8x8-inch baking pan. Drop spoonfuls of the melted peanut butter on top of the brownie batter. Use a knife or a toothpick to gently swirl the peanut butter into the brownie batter, creating a marbled effect.

Place the baking pan in the preheated oven and bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow the brownies to cool in the pan on a wire rack for at least 10 minutes before cutting them into squares.