Mix and bake cake mix as directed for two 8 or 9 inch layers. Let layers cool then split each one in half horizontally to make a total of 4 layers.
In a saucepan over medium heat mix together the white sugar, butter or margarine, and 1/2 cup of the evaporated milk. Cook for 10 minutes. Stir in the marshmallows and mix until smooth. Stir in the coconut. Spread the filling between the cake layers.
Mix together the cocoa, remaining 1/4 cup evaporated milk, the confectioner's sugar, and the vanilla. Beat until smooth and spread over top and sides of cake.