To make this wonderful rainbow pinata cake first you have to bake two semicircular cake sponges. Scoop out the centre of both the sponges and fill the lower one with candies. Join the upper semicircle to the lower with frosting in between. Then divide your buttercream frosting in three to four parts depending upon the number of food colors available. Color each part with a different food color and fill the piping bag with dollops of alternate colored frostings. Make sure not to mix them. Pipe out rosettes on the cake and voila! Your rainbow pinata cake is ready.