Prepare frosting; cover and chill. Place white chocolate in a bowl. Pour boiling water over white chocolate; stir until smooth. Set aside, and let cool.
Grease 3 (9") round cakepans; line with wax paper. Grease and flour wax paper. Set aside.
Beat butter at medium speed with an electric mixer until creamy; gradually add sugar, beating well. Add egg yolks, 1 at a time, beating after each addition until blended. Stir in white chocolate and vanilla.
Combine buttermilk and soda. Add flour to butter mixture alternately with buttermilk mixture, beginning and ending with flour. Mix at low speed after each addition until blended.
Beat egg whites at high speed until stiff peaks form. Gently fold into batter. Pour batter into prepared pans.
Bake at 350° for 20 to 22 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove from pans, and cool completely on wire racks.
Spread Butter-Pecan Frosting between layers and on top and sides of cake. Garnish, if desired. Store cake in refrigerator.