Step 3
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.