Hot Fudge Sauce

In a medium saucepan, melt butter over medium. One by one, whisk in corn syrup, sugar, then cream. Increase heat to medium-high. Whisk until sugar is dissolved and mixture bubbles enthusiastically, about 2 minutes. Add chocolate and whisk until melted. Continue whisking until sauce is glossy and thick, about 30 seconds. Remove from heat. Whisk in vanilla. Serve immediately over ice cream. Or pour fudge sauce into a heatproof container (such as a 1-pint Mason jar). Let cool, then cover and refrigerate. To reheat: Remove lid and microwave in 20-second bursts, stirring between zaps, until hot.