Preheat your oven to 350°F (180°C). Grease a 9×13-inch baking pan, or a jelly roll pan 10.5 x 15.5-inch. Line with parchment paper. In a large mixing bowl, whisk together the melted butter and cocoa powder until smooth.
For The Chocolate Frosting: In a medium-sized mixing bowl, using a hand mixer, beat together the softened butter, milk, cocoa powder, powdered sugar, and vanilla extract until smooth and creamy.