Preheat oven to 350 degrees.
In heavy saucepan on low heat, melt broken chocolate with water, stirring until smooth; set aside.
Cream butter with sugar until light and fluffy.
Add egg yolks, one at a time, beating after each addition.
Mix in melted chocolate and vanilla.
Sift flour with soda.
Add dry ingredients alternately with buttermilk to chocolate mixture.
Mix until smooth.
Beat egg whites with salt until very stiff peaks form; fold into batter.
Spread into three 8 or 9 inch round cake pans lined with wax paper.
Bake 30 - 35 minutes.
Cool on rack 10 minutes; remove cake.