Preheat oven to 350F (177C) and lightly grease/spray a 13×9 pan (or line with parchment paper).

In a large, microwave-safe bowl, heat butter in microwave until melted.12 Tablespoons (170 g) salted butter

Stir in cocoa powder and sugar until well combined.¾ cup (75 g) natural cocoa powder, 1 ⅔ cup (345 g) granulated sugar

Add eggs, egg yolk, and vanilla extract and stir until well combined.2 large eggs + 1 large egg yolk, 1 teaspoon (5 ml) vanilla extract

In a separate, medium-sized bowl, combine flour, baking powder and salt.1 ⅓ cup (165 g) all-purpose flour, ½ teaspoon (2 g) baking powder, ¼ teaspoon (2 g) salt

Gradually add to butter mixture until completely combined.

Spread batter evenly into prepared pan and bake on 350F (177C) for 25 minutes. Allow to cool completely before covering with peanut butter fudge.

Peanut Butter LayerCombine peanut butter and butter in large heat proof bowl and microwave at 15-second increments (stirring between each) until butter is completely melted and mixture is smooth.2 cups (475 g) creamy peanut butter, ½ cup (113 g) salted butter

Stir in vanilla extract.1 teaspoon (5 ml) vanilla extract

Gradually stir in powdered sugar until completely combined. If mixture is too stiff, add milk. 2 ¼ cups (285 g) powdered sugar, 1 Tablespoon (15 ml) milkMixture should be fudgy enough that you can mold pieces into a ball, but not too stiff that you won't be able to spread it over the brownies (feel free to add additional milk, 1 teaspoon at a time, if mixture is too stiff, or more powdered sugar, 1 Tablespoon at a time if mixture is too soft).

Drop by heaping spoonfuls over the cooled brownies and use a knife to gently spread until the brownies are evenly covered with a thick layer.

Chocolate ToppingPrepare chocolate topping by combining chocolate chips and remaining butter in a medium heat-proof bowl and microwaving at 15-second increments (stirring well between) until completely melted.6 Tablespoons (85 g) salted butter, 1 ½ cups (265 g) semisweet chocolate chips

Evenly pour chocolate over fudge and spread with a knife or offset spatula until smooth.

Allow chocolate to set before slicing and serving (30-60 minutes).