Makes about 80 pieces
* 3 cups packed light brown sugar
* 1 cup heavy cream
* 2 tablespoons unsalted butter
* 1 teaspoon vanilla extract
* 1/4 teaspoon salt
1. Line a 9-inch baking pan with foil, leaving overhang on all sides. Grease the foil. Combine all ingredients in a medium saucepan and bring to a boil over medium-high heat. Reduce the heat to medium-low and cook until the mixture reaches the soft-ball stage on a candy thermometer, about 234 degrees, 5 to 8 minutes. If you do not have a candy thermometer, see note below.
2. Remove the pan from the heat and allow to cool until just warm (about 120 degrees) and not yet firm, 35 to 45 minutes. Using a wooden spoon, stir vigorously until the mixture lightens and is no longer shiny, 6 to 10 minutes. Spread the mixture into the prepared pan and let it sit until firm, about 1 hour. Using the foil overhang, remove the fudge from the pan and cut into 1-inch squares.
3. NOTE: If you do not have a candy thermometer, here's what to do. After the candy mixture has simmered for 15 minutes, you can test the soft-ball stage by using a metal spoon to drop a small dollop of the hot mixture into a bowl of cold water. The mixture will cool instantly, and if you can form the mixture into a soft, pliable ball, the mixture has reached the correct temperature. If the mixture stays loose in the water and will not form a ball, it will need more cooking—check it every few minutes. On the other hand, if the mixture instantly seizes in the cold water into a hard, rock-like piece, it is overcooked and you must start over again. That's why it makes sense to start checking the fudge early—and often—or invest in a candy thermometer.