Prep the cake. Start with a round cake, approx. 25 cm in diameter. I used my plain cake recipe scaled by 1.5 times. Use a knife or cake leveler to level the cake.
Make the buttercream. Beat the butter until as white as possible, add the vanilla, and one-half of the icing sugar. Beat until well combined. Add one-half of the milk. Beat. Add the remaining icing sugar and beat until combined. Add the remaining milk and beat. You should have a nice soft consistency buttercream.
Funfetti buttercream. Take one-third of the buttercream and put it into a separate bowl, stir through the sprinkles so that you end up with funfetti butter cream.
Ice the cake. Colour the remaining buttercream and add blue food colouring until you achieve a sky blue colour. Ice the cake, I like to do a 2 stage process. I roughly ice the cake (crumb coat) and then pop in the fridge to chill for a little while. I then add a second layer of buttercream so that I can get a nice even finish with no crumbs. Save some of the blue buttercream to ice the backside of the cake, which will be visible when you stand it up on the serving plate.
Prep the decorations. Trace a small circle onto a piece of baking paper. Mine had a diameter of 6cm. Cut the circle out and fold in half, this will be your guide for making the jelly bean rainbow.
Sort the jellybeans by colour. I used 6 colours to form my rainbow. Orange, Yellow, Green, Blue, Purple and Pink.
Make the marshmallow clouds. Using scissors or a sharp knife cut the marshmallows in half. I use white 3 marshmallows per cloud. Arrange the marshmallow halves into a cloud shape, have the sticky cut overlap them a little and roll with a rolling pin. They should stick together like clouds.
Decorate the rainbow cake. Smooth off the buttercream with a hot knife or spatula. Place the half round piece of baking paper in the centre of the cake along the flat edge. Use this as a guide for placing the marshmallow clouds and the jelly bean rainbow. Remove the piece of baking paper when complete
Finish the cake. Carefully transfer the cake onto a serving plate, stand the cake so that it sits up on the serving plate. Ice the backside of the cake with blue buttercream. Store in the fridge for up to 48 hours before serving. It's best to remove it from the fridge for 1-2 hours before serving so that the buttercream softens again.