Preheat oven to 350F and spray a 13x9 pan with baking spray
Combine butter and chocolate chips in a large, microwave-safe bowl.
Microwave for 30 seconds. Stir well, microwave for another 15 seconds and then stir well again. Repeat for 15 seconds until chocolate and butter are completely melted and well-combined.
Add cocoa powder and stir well.
Add sugars, stir until completely combined.
Add eggs and then egg yolk, one at a time, stirring well after each addition
Add vanilla extract and salt, stir to combine.
Add flour, stir until completely combined.
Spread approximately 1/2 of the batter evenly into prepared pan and make your peanut butter fudge
Combine peanut butter chips, condensed milk, and peanut butter in a medium-sized saucepan over medium-low heat.
Stir frequently until peanut butter chips are melted and mixture is smooth.
Remove from heat, stir in vanilla extract and salt, and pour evenly over brownie batter in pan, use a spatula to spread evenly, if necessary.
Allow to set for about 5 minutes, and then spread the remaining batter over the fudge, doing your best to cover completely with brownie batter.
Bake on 350F for 40-50 minutes, use a toothpick to check for doneness at 40 minutes (toothpick should come out with few fudgy moist crumbs, but not wet batter).
Allow to cool completely before cutting and serving. The longer you allow the brownies to set the firmer the fudge will become.
*For a stronger peanut butter flavor, you may use peanut butter chips instead and omit the actual peanut butter. Peanut butter chips can be difficult to find, which is why I wrote this using white chocolate chips.