Prepare cake mix according to package directions, making two 8 or 9-inch layers; when completely cool, split both layers. Combine sugar, sour cream and coconut, blending well; chill. Reserve 1 cup sour cream mixture for frosting; spread remainder between layers of cake which are placed split side up. Combine reserved sour cream mixture with whipped topping; blend until smooth. Spread on top and sides of cake. Seal cake in an airtight container, and refrigerate for three days before serving. (It keeps real well--if it lasts that long!