Combine buttermilk, eggs, and vanilla in a large mixing bowl; beat at medium speed of an electric mixer until smooth. Set aside.
Combine flour, sugar, soda, and salt; gradually add to buttermilk mixture, beating well after each addition.
Melt chocolate and butter in top of a double boiler. Fold chocolate mixture into batter. Pour batter into 3 greased and floured 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 layers from pans; let cakes cool completely on racks. Spread Fluffy White Frosting between layers and on top and sides of cake.