Melt chocolate in boiling water and cool.
Cream butter and sugar until fluffy.
Add yokes one at a time, beating well after each.
Blend in vanilla and chocolate.
Sift flour with soda and salt; add alternatively with buttermilk to chocolate mixture, beating after each addition until smooth.
Fold in beaten whites.
Pour into three 9-inch layer pans, lined with paper.
Bake at 350°F for 30 to 35 minutes.
Cool and frost tops only.