Heat water and chocolate until melted, stirring occasionally.
Mix flour, baking soda and salt, set aside.
Beat butter and sugar in large bowl with electric mixer on medium speed until light and fluffy.
Add egg yolks, one at a time, beating well after each addition.
Stir in chocolate mixture and vanilla.
Add flour mixture alternately with buttermilk, beginning and ending with flour.
Beat egg whites in another large, clean bowl on high speed until stiff peaks form.
Fold into chocolate batter.
Pour into prepared pans (either 3, 9 inch pans which have been greased, floured and lined with wax paper or one 13? x 9? inch pan, greased and floured. (I use a 13? x 9? it's easier).
Bake in the middle of a preheated oven at 350 degrees for about 30 minutes (about 35-40 for the 13? x 9? inch pan)
Cake will pull slightly from the edges when its done and will spring back slightly when touched in the center.