Make cakes two days in advance. Prepare batter following recipe directions. Bake and cool 6 in., 9 in., 12 in. and 15 in. hexagon cakes. Cakes are 6 in. high (three layers, 2 in. high). Prepare cakes for stacked construction.
Prepare buttercream icing following recipe directions. Use spatula to ice cakes smooth. Position largest cake on prepared base board. Position smaller cakes on cake circle cut to size. Reserve remaining icing.