Preheat your oven to 350°F (175°C) and grease an 8-inch square baking pan or line it with parchment paper for easy removal.
In a medium saucepan over low heat, melt the butter. Once melted, remove from heat and stir in the granulated sugar until well combined.
Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
Sift together the cocoa powder, flour, salt, and baking powder in a separate bowl. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
In a small bowl, mix the creamy peanut butter and milk until smooth.
Pour half of the brownie batter into the prepared pan, smoothing it out evenly. Then, dollop half of the peanut butter mixture on top of the batter. Use a knife or skewer to swirl the peanut butter into the brownie batter.
Pour the remaining brownie batter over the top, followed by the rest of the peanut butter mixture. Swirl again for a marbled effect.
If desired, sprinkle mini chocolate chips over the top for extra chocolate goodness.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the brownies cool in the pan before slicing into squares.
Notes
Dust the brownies with powdered sugar before serving and pair them with a scoop of vanilla ice cream for an indulgent treat!