Hot Fudge Sauce

Hot Fudge Sauce
This makes a really yummy sauce.  It is kind of chewy when put on ice cream.  I have tried a few versions and this is my favorite so far.

*Makes about 4 cups of sauce
1/2 cup granulated sugar
1/4 cup unsweetened cocoa powder
1/4 teaspoon salt
1/2 cup water
1 cup heavy cream
1 cup light corn syrup
1/4 teaspoon distilled white vinegar (weird- but it turned out great)
4 ounces (about 2/3 cup) semisweet chocolate, coarsely chopped and divided- (I just used semisweet chips)
1/4 cup (1/2 stick) butter, softened
1 tablespoon pure vanilla extract
Mix sugar, cocoa and salt in a large, heavy saucepan. Whisk in the water until well blended. Bring to a simmer over medium-high heat, stirring constantly. Remove from the heat and whisk in the cream, corn syrup, vinegar and 2 ounces (about 1/3 cup) of the chopped chocolate.
Return to medium-high heat and boil. Whisk frequently until the bubbles become small and the syrup is thick and sticky, 5-7 minutes (or 225 degrees on a candy thermometer). Remove from the heat and add the rest of the chopped chocolate, butter, and vanilla. Whisk until smooth. Serve at once, or let cool. Cover and refrigerate up to 2 weeks. Reheat over low heat or in the microwave on 50% power.
Recipe Source: from Joy of Cooking