1. Preheat oven to 350°. Grease and flour 3 (8-inch) round cake pans.
2. Pour boiling water over chocolate in a heatproof bowl. Stir until chocolate is melted and smooth. Cool to room temperature (about 30 minutes).
3. Beat butter at medium speed with a heavy-duty electric stand mixer until creamy; gradually add sugar, beating until light and fluffy. Add egg yolks, 1 at a time, beating until blended after each addition. Add melted chocolate and vanilla, beating until blended. Combine flour and next 3 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition.
4. Beat egg whites at medium speed until soft peaks form; gently fold into batter. Pour batter into prepared pans.
5. Bake at 350° for 20 to 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely (about 40 minutes).
6. Meanwhile, prepare Fudge Filling; spread between cake layers. Spread frosting on top and sides of cake.