You can use absolutely any type of thick frosting you like. I've made this using homemade cream cheese frosting, store-bought 'Betty Crocker' style frosting... as long as it's thick enough to hold the decorations, it will work perfectly!
I traced an outline of bunny ears onto a piece of paper, cut it out and used it as a template for cutting the ear shapes out of the sponge (you could also Google and print a bunny ear outline).
This cake is best served on the day. If you need to store it longer, place it in an airtight container in the fridge for up to 2 days (max) and then allow it to come to room temperature before serving.