Preheat your oven to 350°F (180°C). Prepare an 8x8 inch (20x20 cm) pan by lining it with parchment paper. For better adherence, lightly grease the pan.

In a mixing bowl, combine the peanut butter, powdered sugar, and 1 teaspoon of vanilla extract. Mix until creamy and set aside.

Melt the unsalted butter and semi-sweet chocolate together in a heatproof bowl over simmering water. Stir until fully melted and smooth. Allow it to cool slightly.

In a separate bowl, whisk together the granulated sugar and the melted chocolate mixture. Beat in the eggs and the additional egg yolk, followed by another teaspoon of vanilla extract.

In another bowl, sift together the flour, cocoa powder, and salt. Gradually fold this dry mixture into the chocolate mixture until just combined.

Spread half of the brownie batter into the prepared pan. Scoop the peanut butter filling over the batter, spreading it to the edges. Pour the remaining brownie batter over the top, smoothing it out gently.

Bake in the preheated oven for about 25-30 minutes, or until the edges look set. A toothpick inserted in the center should come out with a few moist crumbs.

Allow the brownies to cool completely in the pan before lifting them out and cutting into squares.