Step 1

Preheat oven to 350°. Grease and flour 3 (8-inch) round cake pans.

Step 2

Pour boiling water over chocolate in a heatproof bowl. Stir until chocolate is melted and smooth. Cool to room temperature (about 30 minutes).

Step 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.

Step 4

Beat egg whites at medium speed until soft peaks form; gently fold into batter. Pour batter into prepared pans.

Step 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).

Step 6

Meanwhile, prepare Fudge Filling; spread between cake layers. Spread frosting on top and sides of cake.