Make the cake: Heat oven to 350°. Butter and flour two 9″ cake pans, and set aside. Whisk together flour, baking soda, and salt in a bowl; set aside. Whisk together buttermilk and vanilla in a bowl; set aside. In the bowl of a stand mixer fitted with a paddle, cream butter and sugar on medium-high speed until pale and fluffy, about 3 minutes. Add eggs one at a time, beating well after each addition. On low speed, alternately add dry ingredients in 3 batches and wet ingredients in 2 batches. Increase speed to high, and beat until batter is smooth, about 5 seconds. Divide batter between prepared pans, and smooth top with a rubber spatula; drop pans lightly on a counter to expel large air bubbles. Bake cakes until a toothpick inserted in middle comes out clean, about 35 minutes. Let cakes cool for 20 minutes in pans; invert onto wire racks, and let cool. Using a serrated knife, halve each cake horizontally, producing four layers; set aside.
Make the frosting: Place egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk, and beat on medium-high speed until soft peaks form; turn mixer off. Bring sugar, syrup, salt, and 1⁄2 cup tap water to a boil in a 2-qt. saucepan over high heat, stirring to dissolve sugar; attach a candy thermometer to side of pan, and cook, without stirring, until thermometer reads 250°, 4–5 minutes. Turn mixer to medium speed, and very slowly drizzle hot syrup into beating egg whites. Add vanilla, and increase speed to high; beat until meringue forms stiff peaks and is slightly warm to the touch, about 3 minutes.
To assemble, place one layer on a cake stand, drizzle with 3 tbsp. coconut water, spread with 1 1⁄2 cups frosting, and sprinkle with 1⁄2 cup grated coconut; top with another cake, drizzle with 3 tbsp. coconut water, spread with 1 1⁄2 cups frosting, and sprinkle with 1⁄2 cup coconut. Place another cake over frosting, drizzle with 3 tbsp. coconut water, spread with 1 1⁄2 cups frosting, and sprinkle with 1⁄2 cup coconut; top with remaining cake and drizzle with remaining coconut water. Cover top and sides with remaining frosting, and cover outside of cake with remaining coconut, pressing it lightly to adhere; chill cake to firm frosting. Serve chilled or at room temperature.