Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking pan or line it with parchment paper.
In a medium saucepan, melt the butter over low heat. Once melted, remove it from heat and stir in the sugar until well blended.
Allow the mixture to cool slightly, then whisk in the eggs one at a time, followed by the vanilla extract.
Sift together the cocoa, flour, salt, and baking powder in a separate bowl. Gradually blend the dry ingredients into the wet mixture until just combined.
Pour the brownie batter into the prepared baking pan, smoothing it into an even layer.
In a small microwave-safe bowl, warm the peanut butter in the microwave for about 20 seconds until it’s pourable.
Using a spoon, dollop spoonfuls of the warmed 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.
If desired, sprinkle chocolate chips on top of the brownies for extra flavor.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
Allow the brownies to cool completely in the pan before cutting them into squares.
Notes
Serve with vanilla ice cream or chocolate sauce for an indulgent touch.