Melt chocolate in boiling water; cool. Cream butter with 1 and 2/3 cups of the sugar and beat until light and fluffy.
Add egg yolks one at a time, beating after each addition. Add vanilla to chocolate mixture. Add chocolate mixture to creamed mixture.
Sift together salt, soda and flour. Add alternately with buttermilk to creamed mixture, beating well after each addition until smooth.
Beat egg whites until frothy, then add the remaining 1/3 cup sugar in four additions, beating well until stiff but not dry. Fold into batter.
Pour into three 8-inch (or two 9 -inch) greased pans with paper lined bottoms, and bake at 350 °F for 20 minutes. Cool. Frost with Chocolate Fudge Frosting.