1. To make eyeballs, line a baking sheet with parchment. Make 2 batches Royal Icing and in a small bowl tint 1/2 cup black; transfer to a piping bag fitted with a #5 plain tip. Put remaining icing in a piping bag fitted with an open coupler or 1/2-inch plain tip. Pipe various-sized white circles onto parchment. While icing is still wet, pipe a black "pupil" into the center of each white circle. Allow eyes to dry completely, about 48 hours.
2. Chill one recipe Chocolate Ganache over an ice bath, stirring occasionally, until spreadable like frosting, about 20 minutes. Trim domed tops from cakes. Transfer 1 cake to a 9-inch cardboard cake round; set on a wire rack over a rimmed baking sheet. Spread with a layer of ganache, then add second cake. Frost with an even layer of ganache, squaring off sides and tops.
3. Prepare a second batch of Chocolate Ganache; allow to sit about 15 minutes. Pour over the cake, letting it drip down the sides. Use a spatula to spread ganache if necessary. Chill cake until firm; transfer cake to serving plate and decorate with eyeball candies.