Preheat your oven to 350°F (175°C). Grease and flour an 8x8 inch baking pan. This prevents the brownies from sticking and makes them easy to remove.

In a large bowl, whisk together the melted butter, granulated sugar, cocoa powder, and salt until well combined. The mixture should be smooth and glossy.

Stir in the vanilla extract. Then, add the eggs one at a time, mixing well after each addition. Make sure everything is fully incorporated for that perfect brownie texture.

Gradually add the flour to the wet ingredients, mixing until just combined. Be careful not to overmix, as this can lead to tough brownies.

In a separate small bowl, combine the creamy peanut butter, powdered sugar, and milk. Mix until smooth and creamy. If the mixture is too thick, add a tiny bit more milk, a teaspoon at a time, until it reaches a drizzable consistency.

Pour the chocolate brownie batter into the prepared baking pan, spreading it evenly.

Drop spoonfuls of the peanut butter mixture over the brownie batter. Use a knife or skewer to gently swirl the peanut butter into the chocolate batter. Be careful not to overmix, as you want to maintain the distinct swirl pattern.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached. The edges should be set, but the center should still be slightly soft.

Let the brownies cool completely in the pan before cutting into squares. This allows them to set properly, making them easier to slice.