Melt butter and chocolate at low heat. (If desired, add milk, and then temper in beaten raw eggs and bring up to about 160 degrees)
Place remaining ingredients in a large bowl of the mixer. Add stovetop mixture. Beat at medium speed until all ingredients are well-blended.
Chill overnight or place the mixing bowl in a large pan of ice water. Beat frosting at high speed until it changes color and is of spreading consistency.
Yield: Will frost two 8- or 9-inch layers or a 10-inch tube cake