This is a very easy cake to do. Start with two 6 inch round cakes and stack them. Make your buttercream frosting and color half of it blue (use a recipe that uses all crisco instead of butter if you want your bubbles to be white). Ice the entire cake blue and smooth it out. The bubbles are just dots mad with a #12 round tip, use a coupler with your bag so that you can change to a #5 round tip to make smaller bubbles with. Start on top of the cake in the center and pipe your dots close together covering the entire top of the cake then start to space them out as you move down the cake. Do the bottom border and add a couple of "bubbles" around the edge. Change your tip and fill in with smaller #5 bubbles.