Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang on two sides.

Spread the peanut butter into a layer about 1/2 inch thick on a parchment-lined baking sheet. Freeze until firm, about 30 minutes.

In a medium saucepan, melt the butter and chopped chocolate over low heat, stirring until smooth. Remove from heat and let cool slightly.

Whisk in the sugar, eggs, egg yolk, and vanilla extract until well combined.

Fold in the flour, cocoa powder, and salt until just combined. Pour half of the brownie batter into the prepared pan.

Remove the peanut butter from the freezer and cut it into a square shape that fits inside the baking pan. Place it over the brownie layer.

Top with the remaining brownie batter, smoothing the top with a spatula.

Bake for about 30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.

Allow the brownies to cool in the pan on a wire rack before lifting out the parchment paper to remove them. Cut into squares and serve.