How to Make It
Preheat oven to 350°F. Lightly grease 2 8-inch round pans with cooking spray, then line with parchment paper.
Prepare cake mix according to package directions, substituting whole milk for water and melted butter for oil. Divide batter evenly among prepared pans and bake according to package directions. Remove from oven and let cool in pans 10 minutes. Invert cakes onto a wire rack; remove parchment paper, and let cool completely about 1 hour.
Prepare chocolate frosting: Beat butter with an electric mixer on medium speed until smooth, about 2 minutes. Add powdered sugar, cocoa powder, heavy cream, salt, and vanilla extract. Beat on low speed 30 seconds, then increase to high speed and beat until smooth and just beginning to lighten in color, about 1 minute. Spread frosting on cake layers and decorate as desired.