In measuring cup with a spout, combine the cream, butter, and salt. Set aside.
In a medium saucepan, over medium heat, cook the sugar, corn syrup, and water, stirring, until the sugar is dissolved. Increase heat to high and bring the mixture to a full rolling boil. Cook until a dark amber color, about 1 to 2 minutes, without stirring. Remove the pan from the heat and carefully pour the cream mixture into the sugar mixture. (Take care, since the sugar will bubble up when the cream is added.) Whisk until smooth.
Put the chocolate in a large heatproof bowl. Pour the hot cream/sugar mixture over the chocolate. Add the vanilla and whisk until smooth. Serve the hot fudge sauce warm.
Store the sauce in an airtight container in the refrigerator for up to 2 weeks. Reheat over low heat before serving.
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