Bake cakes in two 9-inch pans per box instructions; cool completely. Trim domes so layers are flat.
Line a 9-inch cake ring with plastic wrap. Place bottom cake layer inside ring.
Soften ice cream until spreadable. Spoon into center, leaving a 1-inch gap to the ring edge. Freeze until firm, about 2 hours.
Add second cake layer to top, wrap, and freeze overnight.
Heat cream until simmering, pour over chocolate, stir until smooth for ganache. Cool slightly.
Unmold, remove wrap, frost with whipped cream. Pour ganache over top to create drips. Add sprinkles.
Keep frozen until 10–15 minutes before serving.