Preheat oven to 350F. Grease three 9-inch cake pans.
To prepare cake, combine buttermilk and baking soda; mix well. Beat egg whites with a mixer at high speed until stiff.
Combine sugar, butter and shortening; beat with a mixer at medium speed until light. Add egg yolks, one at a time, beating well. Add buttermilk mixture alternately with flour. Stir in vanilla. fold in egg whites. Stir in coconut. Pour into pans.
Bake 18-20 minutes, until a wooden pick comes out clean. Cool in pans on a wire rack for 10 minutes. Remove pans and cool completely on wire rack.