Fudge Glaze

1 cup heavy (whipping) cream1 cup sugar3 tablespoons light corn syrup2 tablespoons unsalted butter, cut into chunks 1/8 teaspoon salt4 1/2 ounces unsweetened chocolate, broken up or coarsely chopped 1/2 teaspoon vanilla extractHot water, if neededIn a heavy 2 1/2 to 3-quart saucepan, combine the cream, sugar, corn syrup, butter and salt. Bring just to a full boil over medium-high heat, stirring with a long-handled wooden spoon. Immediately cover the pot with a tight-fitting lid and boil for 2 minutes to allow steam to wash the sugar crystals (which might cause graininess) from the pan sides. Meanwhile, completely wash any sugar from the spoon.Remove the lid and reduce the heat to medium. If any sugar remains, wash down the pan sides using a pastry brush dipped in warm water or a damp paper towel. If using a candy thermometer to gauge doneness, immediately clip it to the pan, with the tip fully submerged but not touching the pan bottom. Gently boil, stirring and occasionally scraping the pan bottom until the mixture reaches 227 to 228 degrees; this usually takes only 1 to 1 1/2 minutes. Remove from the heat. (If no candy thermometer is available, cook, stirring gently, for 1 minute. Immediately test for doneness by removing the pan from the heat and dropping a bit of the mixture into ice water. Let the mixture stand for about 15 seconds then press a bit between your fingertips. If the mixture clings together in a soft mass and is sticky, it is done. If it is runny and disperses in the water, cook for 20 to 30 seconds longer and test in the water again. Continue cooking and testing until the right consistency is obtained.)Gently stir the chocolate, then the vanilla, into the mixture just until the chocolate melts and the glaze is completely smooth. If it looks separated and oily (suggesting overcooking), vigorously and thoroughly stir in hot water 1 teaspoon at a time until the glaze smooths out and looks glossy. Set aside for 2 to 5 minutes, stirring occasionally, until the glaze thickens and cools slightly. If the glaze seems too thick to flow and spread easily, thin it with a little more hot water a teaspoon at a time until a pourable and spreadable consistency is obtained. If it is too thin and runny to spread, let cool for a few minutes longer so it can stiffen further.Makes enough for a pan of brownies or a 9-by-13-inch cake. "The All-American Dessert Book" by Nancy Baggett (2005 Houghton-Mifflin, $35)