2Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan or line it with parchment paper.
In a medium saucepan, melt the unsalted butter over medium heat. Remove from heat and stir in the granulated sugar, eggs, and vanilla extract until well combined.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Pour the brownie batter into the prepared baking pan and spread it out evenly.
Drop spoonfuls of peanut butter over the brownie batter. Use a knife to gently swirl the peanut butter into the batter, creating a marbled effect.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool completely in the pan before cutting into squares and serving.