Combine flour, soda, and coca. Sift mixture 3 times. In a separate bowl cream shortening. Add sugar to shortening and cream until light and fluffy. Add the eggs, one at a time, and beat for 2 minutes after each addition. Add the flour mixture alternately with the buttermilk, beating at medium speed after each addition. Add the vanilla. Add the water one third at a time, beating well after each addition. Pour the batter into 2 ungreased 20 cm cake pans. Bake in a 350 degree oven for 20 to 25 minutes.
In the top of a double boiler over simmering water, combine the sugar, salt, cream of tartar, egg whites, and water. Beat with an electric mixer at top speed for 4 to 5 minutes. Remove from heat and the simmering water and beat for 1 minute. Add vanilla and blend. Frost the chocolate cake.