Coconut Cake

Heat oven to 350°. In the bowl of a stand mixer fitted with a paddle, beat together sugar and butter until pale and fluffy, 2–3 minutes. Add eggs one at a time, beating well after each addition until smooth. Stir together buttermilk, vanilla, and baking soda mixture, and with the mixer on low speed, add to batter alternately with cake flour, beating until just combined. Pour batter into two buttered and floured 9" round cake pans, and bake until a toothpick inserted in the middle comes out clean, 40 minutes. Let cakes cool completely, and then unmold and split each in half horizontally to create four layers; refrigerate until ready to use.

Crack coconuts, pour coconut water through a fine strainer into a small bowl, and reserve; finely grate coconut meat and set aside. In a 2-qt. saucepan, stir together 2 ¼ cups sugar, corn syrup, salt, and ½ cup water; attach a candy thermometer to the side of the pan and heat over medium-high heat. Meanwhile, place egg whites, vanilla, and cream of tartar in the bowl of a stand mixer fitted with a whisk, and mix on low speed until frothy. Increase speed to medium-high and beat egg whites until soft peaks form. With mixer running, slowly sprinkle in remaining sugar and beat whites until stiff peaks begin to form; turn mixer off. When sugar mixture reaches 248°, return mixer to medium-high speed and slowly drizzle the hot sugar syrup in a small, steady stream into the egg whites, making sure not to let the syrup hit the whisk or side of the bowl; continue beating until stiff peaks form and mixture is still slightly warm, about 3–4 minutes.

To assemble the cake, place one cake layer cut side up on a cake stand and drizzle with some of the reserved coconut water to moisten; spread a ¼"-thick layer of icing on top and then sprinkle with a handful of grated coconut. Repeat with two more layers, and then top with the last layer, cut side down. Spread remaining icing over top and side of cake and then cover with grated coconut, pressing it into the icing lightly to adhere; cover with a cake dome, or loosely with plastic wrap, and refrigerate for at least 2 hours before serving. Serve chilled or at room temperature.