Place a heatproof bowl from a stand mixer over a saucepan of simmering water. (The water should have air bubbles around the edges but not be boiling).
In the bowl, combine the egg whites, sugar, water, and cream of tartar. Cook over medium heat, whisking frequently until the sugar is dissolved and the mixture is foamy, about 5 minutes.
Attach the bowl to a stand mixer fitted with the whisk attachment. Beat the mixture on high speed until it becomes glossy and voluminous. This could take up to 7 minutes. Whisk in the vanilla.
Using a long serrated knife, carefully slice off the top 1 1/2 inches of the cake, making sure to not break it, and set it aside. Cut a channel, 1-inch wide and 1-inch deep halfway between the center and the edge of the cake. Fill the channel with the frosting and spread the frosting over the entire layer and sprinkle the coconut over the layer.
Gently place the reserved cake layer on top of the bottom cake layer. Spread the remaining frosting over the sides and top of the cake.
TOPPING
Sprinkle the coconut nut over the top and sides of the cake, gently patting the coconut into the frosting on the sides.
NOTES
Recipe adapted from Martha Stewart.
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