Fill your cake as desired and give it a crumb coat. Chill.
Frost the outside of your cake with white frosting (or any color you want). Smooth it as much as possible.
Mix your sprinkles or use any color combination you want to press along the bottom of the cake. Do this right after frosting so they'll stick. Do it over the sink too - it'll make a mess! Chill to set.
Drizzle the cake with the chocolate ganache. Let set before decorating.
Use a 1M tip to pipe rosettes around the edge of the cake. Decorate every other one with two yellow dots (Mickey) and white pearl sprinkles (Minnie). Top each with two mini Oreos to represent ears.