PreparationMake the Nut Butter Layer Preheat oven to 350°F. Line an 8x8" baking dish with 2 overlapping pieces of parchment paper, leaving at least a 2" overhang on 2 sides. Whisk eggs, brown sugar, salt, and vanilla in a medium bowl until well combined. Using a rubber spatula, stir in peanut butter until a smooth, thick batter forms. Scrape into prepared baking dish and spread in an even layer. Make the Cocoa Layer and Assemble Using an electric mixer or a stand mixer fitted with the whisk attachment on medium-high speed, beat eggs, granulated sugar, and brown sugar in a large bowl until mixture is light, smooth, and doubled in volume, 3–4 minutes. With mixer running on low speed, slowly beat in butter, then add vanilla and salt and beat to combine. Add cocoa powder and cornstarch. Beat on low speed, scraping down sides of bowl as needed, until incorporated. Increase speed to medium-high and beat until mixture is thick and holds its shape, about 30 seconds. Pour over nut butter layer and smooth top. Add your choice of toppings or leave top smooth—they’re all good options. Bake brownie until top is puffed, glossy, and starting to crack, and the center is just set (it shouldn’t jiggle when shaken, but the chocolate layer will still be very gooey), 25–30 minutes. Let cool. Remove brownie from pan using parchment overhang, then cut into 16 squares.