Dissolve soda in buttermilk. Combine flour, salt, and cocoa; add to creamed mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Mix well after each addition. Stir in coffee and vanilla.
Beat egg whites (at room temperature) until stiff peaks form. Gently fold into the creamed mixture.
Pour into 3 greased and floured 8-inch round cake pans. Bake at 350° for 30 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove from pans, and let cool completely.
Spread Mocha Frosting between layers and on top of cake.