Preheat your oven to 350°F and generously grease a 13×18 jelly roll pan with butter or Baker’s Joy nonstick spray.

In a large bowl, whisk together the melted (and slightly cooled) butter, whole milk, sour cream, and granulated sugar. Add in the eggs and vanilla extract and whisk until fully combined. Sift in the flour, cocoa powder, baking soda, and salt. Whisk again until smooth and lump-free.

Pour the batter evenly into the prepared pan. Bake for 20 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool for 10 minutes before moving on to the next layer.

Using a stand mixer or hand mixer, cream together the peanut butter and softened butter until smooth. Add in the vanilla extract and salt. Gradually add the powdered sugar, ½ cup at a time, beating well between each addition. Once all the sugar is added, the mixture may look a bit dry—don’t worry! Add milk one teaspoon at a time and beat until the frosting is fluffy and spreadable. Spread a thick, even layer over the slightly cooled cake using a spatula.

In a small saucepan over medium heat, combine the melted butter, milk, corn syrup, and vanilla extract. Stir until everything is heated through. Lower the heat, then add the chopped dark chocolate. Stir until the chocolate is fully melted and the mixture is silky smooth. Remove from heat and whisk in the sifted confectioners’ sugar. Let the frosting sit for 5 minutes to cool slightly and thicken just a bit.

Slowly pour the warm chocolate glaze over the peanut butter layer. Use a spatula to gently push it toward the edges to ensure full coverage. Allow the frosting to set for at least 30 minutes before slicing and serving.