Preheat and Prep
Preheat your oven to 350°F (177°C). Lightly grease or spray a 13×9 baking pan. For easier removal and cutting later, you can also line the pan with parchment paper.

Make the Brownie Batter
In a large, microwave-safe bowl, melt the 12 tablespoons of salted butter. Stir in the cocoa powder and granulated sugar until well combined. Add the eggs, extra egg yolk, and vanilla extract, stirring until the mixture is smooth.

Add Dry Ingredients
In a separate medium bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mixture to the wet butter mixture, stirring until completely combined.

Bake
Spread the batter evenly into the prepared pan. Bake for 25 minutes. Crucial Step: Allow the brownies to cool completely before attempting to add the peanut butter layer.

Make Peanut Butter Layer
Combine the creamy peanut butter and 1/2 cup salted butter in a large heat-proof bowl. Microwave in 15-second increments, stirring between each, until the butter is melted and the mixture is smooth. Stir in the vanilla extract.

Thicken the Fudge
Gradually stir in the powdered sugar until completely combined. The texture should be fudgy enough to hold its shape but soft enough to spread. If it is too stiff, add 1 tablespoon of milk. If too soft, add a little more powdered sugar.

Spread the Layer
Drop heaping spoonfuls of the peanut butter mixture over the cooled brownies. Use a knife or offset spatula to gently spread it into an even layer.

Make Chocolate Topping
Combine the chocolate chips and remaining 6 tablespoons of salted butter in a medium heat-proof bowl. Microwave in 15-second increments, stirring vigorously between each, until completely melted and glossy.

Finish and Set
Pour the chocolate ganache evenly over the peanut butter layer and smooth it out. Allow the chocolate to set for 30 to 60 minutes before slicing and serving.