Make The Cake

1. Preheat oven to 350°F. Grease and line a 13 x 9-inch baking pan with parchment paper for easy removal.
2. In a large mixing bowl, combine the cake mix, eggs, extra yolk, melted butter, whole milk, sour cream, vanilla, and espresso powder, if using. Mix until smooth and fully combined. Do not overmix, but make sure there are no dry streaks.
3. Pour the batter into the prepared pan and smooth the top evenly.
4. Bake according to the box directions for a 13 x 9 pan, usually about 28 to 35 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs.
5. Let the cake cool completely before frosting.

Make The Frosting

1. In a large bowl, beat the softened butter until creamy and smooth.
2. Add powdered sugar and cocoa powder. Mix on low until combined.
3. Add heavy cream one tablespoon at a time until the frosting reaches a fluffy, spreadable consistency. Beat in the vanilla.
4. Whip for another 1 to 2 minutes until light and smooth.

Assemble

1. Spread the frosting evenly over the cooled cake using an offset spatula.
2. Finish with a generous sprinkle of rainbow candy chips for that bold confetti contrast against the dark chocolate base.
3. Slice and serve.