This Lemon and Grapefruit Buttermilk Cake is one of the absolute best-tasting cakes I've made. It's infused with lemon zest and the juices of both grapefruit and lemon. A grapefruit glaze is poured over the top of the cake after it's baked which gives the cake an incredibly moist crumb. To further enhance the bold citrus flavor, a lemon icing is drizzled over the top.