Preheat your oven to 350F (180C). Grab an 8x8 inch (20x20 cm) baking pan and line it with parchment paper. Lightly grease the pan first for easier removal.

In a medium bowl, combine the smooth peanut butter, powdered sugar, and 1 teaspoon of vanilla extract. Blend until smooth and creamy.

In a microwave-safe bowl, add the unsalted butter and semi-sweet chocolate. Microwave in short bursts of about 30 seconds, stirring in between until melted and combined.

Once the chocolate mixture has cooled slightly, stir in the granulated sugar until well combined. Then add in the two large eggs and one egg yolk along with another teaspoon of vanilla extract. Mix until smooth.

Sift together the all-purpose flour, cocoa powder, and salt into your chocolate mixture. Gently fold until just combined; do not overmix.

Pour half of the brownie batter into your prepared pan and spread it evenly. Dollop spoonfuls of the peanut butter filling on top, then pour the remaining brownie batter over the peanut butter layer.

Bake in the preheated oven for about 35 minutes or until a toothpick inserted comes out with just a few moist crumbs attached.