Using a serrated knife, trim domes from 8-inch round cakes. Spread about 3/4 cup frosting over top of one cake layer, then top with second 8-inch cake. Use about 2 1/4 cups of frosting to coat tops and sides, spreading smooth with a spatula. Repeat with 6-inch cake layers, using about 1/2 cup frosting for the filling and about 1 1/4 cups frosting for the tops and sides. Spread nonpareils over the top of the 8-inch cake, being careful to keep them from falling down the sides. Refrigerate cakes until firm.
Cut a chocolate wafer cookie in half to create the two ears. Trim two wafers with a serrated knife, using an egg-shaped cookie cutter as a guide, to form eye patches. Trim one more cookie for a nose.
Transfer 8-inch cake to serving platter using two wide offset spatulas or a large grilling spatula. Carefully position the 6-inch tier on top of the 8-inch tier, and touch up the frosting on the sides if necessary. Position ears on top of cake. Affix eye patches, nose, and gumdrop for the mouth to side of cake, adhering with more frosting if necessary. Use a drop of frosting to attach M&M's to the eye patches.