Melt chocolate in 1/2 cup boiling water; cool. Cream butter and sugar until light and fluffy. Add egg yolks, one at a time, beating after each addition. Add vanilla and melted chocolate and mix until blended.
Sift flour with soda and salt. Add sifted dry ingredients to chocolate mixture alternately with buttermilk, beating after each addition until batter is smooth. Fold in stiffly beaten egg whites.
Line bottoms of three 8- or 9-inch layer cake pans with wax paper. Pour batter into prepared pans. Bake at 350 degrees for 35 to 40 minutes. Cool. Frost top and between layers with Coconut Pecan Frosting.
In a saucepan, combine evaporated milk or half and half, sugar, egg yolks, margarine and vanilla. Cook over medium heat for 12 minutes, stirring constantly until mixture thickens. Add about 1 cup thin flaked coconut and chopped pecans. Beat until cool and of spreading consistency. Makes enough to cover tops of three 9-inch layers. (Do not frost sides of cake.)