Melt unsweetened chocolate in double boiler over, not in, hot water. Add and melt buttter. Stir and blend well, then add boiling water. Stir well. Add sugar and corn syrup. Permit the sauce to boil readily but not furiously over direct heat. Do not stir. Boil 5 minutes for ordinary sauce, boil 8 minutes for sauce that hardens on ice cream. Remove from heat and add vanilla and stir. May reheat leftover sauce over hot water.