Combine chocolate and boiling water; stirring until melted. Set aside.
Cream butter; gradually add sugar, beating well. Add egg yolks, one at a time, beating well after each addition. Add chocolate mixture; beat well.
Dissolve soda in buttermilk. Combine flour and salt; add to creamed mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Stir in vanilla.
Beat egg whites (at room temperature) until stiff peaks form; fold into batter.
Pour batter into 3 waxed paper-lined and greased 9-inch round cake pans. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove from pans, and cool completely.
Spread Coconut-Pecan Filling between the layers and on top of the cooled cake.