Preheat oven to 350℉. Line an 8-inch square baking pan with parchment paper and set aside.
Add melted butter and melted chocolate to a large mixing bowl, whisking well to combine. Add sugar and whisk again, until ingredients are fully incorporated and batter is shiny.
Whisk in the egg, egg yolk, and vanilla extract to create a thick batter. Using a large spoon, incorporate the cocoa powder and flour, stirring just until there are no visible streaks of flour in the batter.
Transfer the brownie batter to the prepared pan, and use an offset spatula or back of a spoon to spread it into an even layer.Using the tip of a butter knife, swirl the peanut butter into the batter.
Set pan on center rack of preheated oven and bake for 25 to 30 minutes, or until a toothpick inserted into the brownies comes out without any wet batter.