How to Make It
MAKE THE CAKE: Preheat the oven to 350°. Butter and flour two 9-inch round cake pans. Sift the cake flour with the baking powder, baking soda and salt. In a large bowl, using an electric mixer, beat the butter with the granulated sugar at high speed until light and fluffy, about 5 minutes. Beat in the eggs, 1 at a time, beating well after each addition.
Stir the vanilla into the buttermilk. Using a large rubber spatula, stir the dry ingredients into the batter in 2 batches, alternating with the butter-milk. Scrape half of the batter into 1 of the prepared cake pans and smooth the surface.
Fold the walnuts, currants, cinnamon, allspice and cloves into the remaining half of the cake batter. Scrape the spice batter into the second cake pan and smooth the surface. Bake the cakes in the middle of the oven for 35 minutes, or until a toothpick inserted in the center of each comes out clean. Transfer the cakes in their pans to wire racks and let cool for 15 minutes, then turn them out onto the racks and let cool completely. Leave the oven on.
MEANWHILE, MAKE THE COCONUT FROSTING: Spread the coconut on a large rimmed baking sheet and bake, stirring occasionally, until golden brown, about 8 minutes. Transfer to a plate and let cool.
In a medium saucepan, bring 1 inch of water to a simmer over moderate heat. In a medium stainless steel bowl, combine the egg whites with the confectioners' sugar, water and cream of tartar. Using clean beaters, beat at medium speed until blended. Set the bowl over the simmering water and beat at high speed until thick and fluffy, about 7 minutes. Remove the bowl from the pan and beat the vanilla and lemon zest into the frosting.
Using a serrated knife, halve each cake layer horizontally. Set 1 plain cake layer on a cake plate and spread with 1 cup of the frosting. Sprinkle 1/2 cup of the toasted coconut over the frosting, then spoon 1/2 cup of the frosting around the edge and in the center of the cake. Top with a spice cake layer. Continue frosting and sprinkling with this and the remaining cake layers, alternating the plain and spice cakes. Spread the top of the cake with the remaining frosting and sprinkle with the remaining coconut. Cut into wedges and serve.
Make Ahead: The 2 uncut cake layers can be wrapped in plastic and stored overnight at room temperature. The frosted cake can be stored at room temperature for up to 4 hours.