Preheat your oven to 350°F (175°C).
Grease and line a baking pan with parchment paper.
Melt the butter in a saucepan over low heat.
Add granulated sugar to the melted butter, stirring until combined.
Remove from heat and let cool slightly before adding eggs, one at a time.
Stir in the vanilla extract and mix until smooth.
In a separate bowl, whisk together the flour, cocoa powder, and salt.
Gradually fold the dry ingredients into the wet mixture until just combined.
Prepare the peanut butter swirl by mixing peanut butter and powdered sugar in a bowl until smooth.
Pour two-thirds of the brownie batter into the prepared pan, then dollop the peanut butter mixture on top.
Add the remaining brownie batter on top of the peanut butter layers.
Use a knife to create swirls through the batter.
Bake for 25-30 minutes or until a toothpick comes out with moist crumbs.
Let the brownies cool completely in the pan before slicing.
