I've made two layer (round) vanilla cake (with a splash of extra vanilla in it). Frost it with a green tinted frosting.. like whipped cream or cool whip. Put a small swirl of white cool whip/whipped cream in the top center (like with piping bag and shell decorating tip). Poke a couple nice St. Patty's Day decorations into the top (sometimes you can get those from a good florist).. maybe a hint of green sugar crystals, or that edible glitter sprinkled around the top..........and poke some of those gold foil covered chocolates into the sides.