Preheat the oven to 350°F. Spray two 8-inch round pans with baking spray and line the bottoms with parchment paper.
In a large bowl, mix the cake mix, milk, oil, eggs, and sour cream until smooth. Divide evenly between the pans and bake according to the box directions. Let the cakes cool completely.
To make the frosting, beat the butter until smooth and creamy. Add the powdered sugar, cocoa powder, vanilla, and heavy cream, mixing until fluffy and spreadable.
Place one cake layer on a cake board or plate. Spread an even layer of chocolate frosting on top. Add the second cake layer, flipping it so the flat bottom faces up. Frost the top and sides of the cake, adding a second coat if needed and reserving a little frosting for decorating.
Press rainbow sprinkles around the bottom edge of the cake. Transfer the remaining frosting to a piping bag fitted with a small star tip and pipe a thick border around the top edge. Sprinkle nonpareils over the border, then lightly dust the center of the cake with glittery sugar. Finish with a New Year’s topper.