Step 1 This sauce is foolproof so long as you heat it slowly: In the top of a double boiler over (and touching) barely simmering water, combine the chocolate, cream, sugar, corn syrup and water. Stir frequently until all the chocolate has melted, then stir occasionally until the sauce is thick and glossy and is between 160 and 165 degrees (the exact temperature is not critical so long as you are close), 15 to 20 minutes (going slowly is the key here). Remove from the heat.
Step 2 Serve the sauce immediately, or set it aside until needed. lt can be kept, covered and refrigerated, for at least a week or frozen for 3 months. Reheat in a double boiler or microwave on medium (50%) power, using short bursts and stirring frequently. Do not simmer or boil.