Preheat oven to 350 degrees F or 325 degrees F for a dark, nonstick pan. Grease the bottom of a 9x13-inch baking pan.
In the bowl of a stand mixer or using a large mixing bowl with a hand mixer, add the sugar, melted butter, cocoa powder, and vanilla. Beat on medium speed until combined.
Add the eggs and beat until fully incorporated.
Add in flour and salt. Mix on low until just combined.
Spread the brownie batter evenly in the prepared pan using an angled spatula. Place in the preheated oven and bake for 30-35 minutes or until a toothpick inserted in the center of the brownie comes out clean or moist crumbs. Cool completely.
Peanut Butter Frosting
In a medium bowl, add the butter, peanut butter, powdered sugar, and milk. Beat with a hand mixer on low speed until most of the powdered sugar is combined. Scrape down the sides of the bowl and turn mixer up to medium speed and beat for an additional minute until creamy and fluffy.
Add dollops of frostingon top of the cooled brownies. Using an angled spatula, spread filling evenly over brownies.
Place in the fridge and chill until the frosting is firm.
Chocolate Topping
In a medium microwavable bowl, add the chocolate chips and sliced butter. Microwave uncovered on high for 30 seconds and then stir. Continue microwaving in 30-second intervals and stirring afterward until the chocolate chips are completely melted and creamy. If the chocolate seems too buttery at first, keep stirring until it’s smooth and well combined. Let the chocolate cool for 5 to 10 minutes. Add the 1 tablespoon of heavy cream to the chocolate and stir until combined. It should be a spreadable consistency. You can add more heavy cream a tablespoon at a time to thin it out to a pourable consistency if you would prefer.
Spread the chocolate topping on top of the chilled frosting using an angled spatula.
Cover with plastic wrap and place in the fridge for 30 to 45 minutes until the chocolate is set.
Slice into 12 brownies and serve.
Store in an airtight container or tightly wrapped in plastic wrap in the refrigerator for up to 1 week.
Notes
Be sure to completely cool the cake before adding the peanut butter layer to prevent the peanut butter itself from melting.Make sure that the chocolate topping is thin enough to spread easily. If the topping is too thick it can lift up the peanut butter layer. You can thin the topping by adding more milk to it.To ensure cleaner cuts, make sure that the brownies are first chilled and then sliced with a sharp knife.