Vivid rainbows that lead to a bountiful stash of gold are part and parcel of Irish folklore. What better way to celebrate St. Patrick's Day than with this showstopping layered rainbow cake with a gold ombre center? Start with the batter from this Classic White Cake recipe and this simple vanilla buttercream recipe, doubled. You'll also need: red, yellow and blue food coloring; gold sprinkles; a rotating cake stand or lazy Susan; three 8-inch round cake pans; a cooling rack; pastry bags or plastic baggies; rubber bands; a small offset spatula; scissors; a bench scraper or a large offset spatula.