In a large microwave-safe bowl, melt butter and chocolate; stir until smooth. Stir in sugar. Add eggs, one at a time, beating well after each addition. Stir in vanilla, flour and salt.
Pour into three greased and floured 9-in. round baking pans. Bake at 350° for 23-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes; remove from pan to a wire rack to cool completely.
For frosting, melt chocolate in a heavy saucepan over medium heat. Gradually stir in cream and sugar if desired, until well blended. Heat to a gentle boil; boil and stir for 1 minute. Remove from the heat; transfer to a large bowl. Refrigerate for 2-3 hours or until mixture reaches a pudding-like consistency, stirring a few times.
Beat until soft peaks form. Immediately spread between layers and over top and sides of cake. Sprinkle with shaved chocolate. Store in the refrigerator.
