1. Preheat oven to 350 degrees. Grease bottom of three 9" layer pans with wax paper. 2. In saucepan, melt chocolate over boiling water (in double boiler). Let cool. 3. In small bowl, mix flour, baking soda and salt. Set side. 4. In large bowl (with electric mixer), cream butter and sugar until light and fluffy. Beat egg yolks and vanilla. Stir in chocolate. Add flour mixture alternately with buttermilk, beginning and ending with flour mixture, beating well. 5. In separate bowl, beat egg whites until stiff peaks form. Fold into batter gently and thoroughy. 6. Divide batter among the 3 pans- smoothing tops. Bake in pre-heated oven for 30 minutes or until cake springs back when touched in center. Cool cake pans on racks for 15 minutes. Remove cakes from pans and peel off wax paper. Cool cakes complety on racks before frosting.