Preheat oven to 350°F. Grease two 9-inch round pans and line bottoms with parchment.
Mix cake batter per boxed directions, using milk, eggs, and oil. Divide evenly into pans.
Bake 25–30 minutes, until a toothpick comes out clean. Cool 15 minutes in pans, then on a rack.
Make black buttercream by tinting 4 cups with black gel. Stir in luster dust for sparkle.
Stack cakes with a thin layer of buttercream between. Coat whole cake in an even crumb coat. Chill 20 minutes.
Finish with final black coat. Thin small amounts of neon gels with a tiny bit of clear alcohol and splatter for galaxy streaks.
Pipe small stars with royal icing. Add glow drops per package directions and test under a black light.