In a medium saucepan, combine the brown sugar, granulated sugar, heavy cream, corn syrup, and salt and heat over medium heat, stirring to dissolve e the sugar. Insert a candy thermometer and cook, stirring occasionally, until the mixture reaches 238 F (this is known as the soft ball stage: see Note), which will take about 10 to 15 minutes. Be careful that the mixture doesn’t boil up and over the sides of the pan; a little bubbling is fine.
Add the butter and stir until melted and blended in, then remove the pan from the heat and let it stand without stirring at all until the mixture has cooled to lukewarm, about 120 F).
Beat in the vanilla until the mixture is smooth and creamy. Fold in the nuts, if using. Pour the fudge into the prepared pan and let cool completely. If your kitchen is quite warm, you might want to let it cool in the fridge.
Once the fudge has set, cut it into 1-inch pieces with a thin sharp knife. Store in an airtight container for up to a week or in the fridge if it’s not all that cool in your kitchen. Let come to room temp just before serving.
Notes
Soft-ball stage refers to a narrow temperature range used when cooking sugar syrups, specifically between 235 degrees and 245 degrees F. This is most easily and accurately measured with a candy thermometer, BUT if you don’t have a candy thermometer, you can still determine when this temperature range has been reached. Fill a small bowl with very cold water. Drop a very small spoonful of the hot syrup into the water. Stick your fingers into the water and gather the cooled syrup into a ball. If it has reached soft-ball stage, the syrup should easily hold its ball shape, though it should squish when removed from the water.
Brown sugar fudge or penuche will last for 2 weeks in a tightly sealed container at room temperature or in the fridge. It can also be frozen for up to 4 months, tightly wrapped.