Butter the sides of a heavy 2 quart saucepan. In it combine the first 4 ingredients. Cook and stir over medium-low heat till mixture boil.
Cook to soft ball stage(238^), stirring only to prevent sticking(mixture should boil gently over entire surface).
Remove from heat;add butter and vanilla but do not stir. THIS IS AN IMPORTANT STEP. Cool, without stirring, to lukewarm(110^), about 55 minutes.
Then beat fast about 10 minutes or till mixture loses its shine.
Spread in a buttered loaf pan 8x4-inch. Score while warm; cut when firm.
Cherry Opera Fudge:
Make as above, except stir in 1/4 cup or more chopped candied cherries,before spreading into loaf pan.
Almond Opera Fudge:
Prepare as above ,except add 1/4 teaspoon almond extract with vanilla; stir in 1/3 or more chopped toasted almonds before spreading into loaf pan.