This recipe is based on David Lebovitz’s fudge sauce in his ice cream bible, The Perfect Scoop. It has the perfect balance of dark chocolate depth and sweetness, with a terrific texture. Don’t spoon this on the sundae until it cools off and is barely warm, otherwise it will melt the ice cream too quickly.
In a 1 ½-2 quart heavy saucepan, mix the cream, brown sugar, cocoa, and corn syrup. Bring to a boil over medium heat, stirring constantly. Immediately remove from the heat and add the chocolate and butter. Stir until the chocolate is melted and the sauce is smooth, then add the vanilla. Cool the sauce to lukewarm, or refrigerate and rewarm at low heat on the stove or in the microwave. The sauce keeps in the refrigerator for several weeks.