3 cups brown sugar, lightly packed
1 cup heavy cream
2 tablespoons unsalted butter
1 teaspoon vanilla extract
14 teaspoon salt
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 235F, 5-8 minutes. Remove pan from heat and allow to cool just until warm (about 120F) and not yet firm, 35-45 minutes. Using a wooden spoon, stir vigorously until the mixture lightens and is no longer shiny, 6-10 minutes. Spread mixture into the prepared pan and let set until firm, about 1 hour. Using the foil overhangs, remove the fudge from the pan and cut into 1-inch squares.