1. Preheat the Oven:
Preheat your oven to 350°F (175°C). Grease and line an 8×8-inch square baking pan with parchment paper or cooking spray.

2. Make the Brownie Batter:
In a large bowl, whisk together the melted butter and granulated sugar until smooth. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

3. Mix the Dry Ingredients:
In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder. Gradually add the dry ingredients to the wet ingredients, stirring until fully combined and smooth.

4. Prepare the Peanut Butter Swirl:
In a small bowl, combine the creamy peanut butter, powdered sugar, and milk. Stir until the peanut butter mixture is smooth and slightly runny. You can adjust the amount of milk depending on how thick or thin you want the peanut butter to be.

5. Layer the Brownie and Peanut Butter Mixtures:
Pour the brownie batter into the prepared pan, spreading it out evenly. Drop spoonfuls of the peanut butter mixture over the brownie batter, then use a knife or a toothpick to swirl the peanut butter into the brownie batter, creating a marbled effect.

6. Bake:
Bake for 25-30 minutes, or until a toothpick inserted into the center of the brownies comes out with a few moist crumbs (they should still be fudgy in the middle). Keep in mind that baking time can vary depending on your oven and the thickness of your brownies.

7. Cool and Serve:
Allow the brownies to cool completely in the pan before cutting them into squares. This ensures the peanut butter swirl sets nicely and makes cutting easier.

8. Serve and Enjoy:
Serve the brownies as they are, or pair them with a scoop of vanilla ice cream or a drizzle of chocolate syrup for an extra indulgent treat!