Preheat your oven to 350°F (175°C) and grease a 13x18 inch jelly roll pan.

In a large mixing bowl, whisk together melted butter, whole milk, sour cream, granulated sugar, eggs, and vanilla extract until smooth.

In another bowl, sift together all-purpose flour, cocoa powder, baking soda, and salt. Gradually add to the wet ingredients, stirring until smooth.

Pour the batter into the greased pan and bake for approx. 20 minutes, or until a toothpick comes out clean.

While the cake cools, prepare the peanut butter frosting by beating together creamy peanut butter, softened butter, vanilla extract, and salt until smooth.

Gradually add powdered sugar, mixing well and adjust with milk for texture.

Spread the frosting over the cooled cake and set aside.

In a saucepan, melt additional butter, milk, corn syrup, and vanilla over low heat. Stir in chopped dark chocolate until smooth.

Whisk in powdered sugar until shiny. Allow it to cool slightly, then pour it over the frosted cake.

Let the cake sit at room temperature for at least 30 minutes to set, then slice and serve.