Preheat oven to 350o F.Prepare the cake mix according to package directions.Substituting the milk for water.Divide cake batter among three greased 9-inch round cake pans.Bake for 20 to 25 minutes or until golden.Let cool in pans for 10 minutes.Remove cakes from pans; cool completely on a wire rack.
While the cakes cool, prepare the filling.Whip the heavy cream until stiff peaks form (tips stand straight).Fold in the sour cream, then gradually fold in the confectioners' sugar.Stir in the vanilla.Fold in the sweetened coconut.
Using a serrated knife, split each cake in half horizontally.Spread the filling over the layers, stacking all the layers.Place the cake in an airtight container or cover tightly with plastic wrap.Refrigerate overnight or up to three days before frosting.
Whenyou are ready to frost the cake, make the frosting.In the top of a double boiler or in a metal bowl, combine the granulated sugar, egg whites,water, corn syrup, and salt, beat for 1 minute with anelectric hand mixer.Place the pot or bowl over (not in) a pot of boiling water. Cook, beating constantly with mixer, about 7 minutes or until the frosting becomes fluffy and stiff peaks form (tips stand straight). Remove the pot from the boiling wwater.Add the vanilla: continue beating for 1 to 2 minutes more until barely warm.Frost the top and sides of the cake.Sprinkle the cake with additional coconut.