Preheat oven to 350°F. Grease an 8x8-inch baking pan and line with parchment paper.

In a medium saucepan, melt butter over low heat. Remove from heat and stir in granulated sugar and brown sugar until well combined.

Add eggs and vanilla extract, mixing well after each addition.

Sift together flour, cocoa powder, and salt. Gradually add to the wet ingredients, stirring until just combined.

Pour batter into the prepared pan, spreading evenly.

Dollop peanut butter by teaspoonfuls over the batter. Use a knife to swirl the peanut butter into the batter, creating a marbled effect.

Bake for 30-35 minutes, or until a toothpick inserted into the center comes out mostly clean with a few moist crumbs.

Allow to cool in the pan on a wire rack before slicing into squares.