In heavy saucepan on low heat, melt broken chocolate with water; 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 or coffee 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" round cake pans lined with wax paper.
Bake at 350-F about 30 minutes. Cool on rack 10 minutes; remove cake