Line the 8-inch square baking dish with parchment paper, leaving some overhang for easy removal later. If you don’t have parchment paper, lightly grease the dish with butter.

In a medium saucepan, melt the butter over medium heat. Once melted, add the granulated sugar, brown sugar, heavy cream, corn syrup, and salt.

Stir the mixture until it is well combined and starts to boil. Attach the candy thermometer to the side of the saucepan, ensuring it does not touch the bottom.

Continue to cook the mixture, stirring constantly, until it reaches the soft-ball stage (234°F to 240°F).

Once at the desired temperature, remove the saucepan from the heat. Allow the mixture to cool for about 5 minutes.

After it has cooled slightly, add the vanilla extract and dark rum. If using nuts, fold them in now.

Beat the fudge mixture vigorously with a wooden spoon or silicone spatula until it thickens and begins to lose its shine (approximately 5-7 minutes).

Pour the fudge into the prepared baking dish and spread it evenly with the spatula.

Allow the fudge to set at room temperature for at least 2 hours or until firm.

Once set, lift the fudge out of the dish using the parchment overhang and cut it into squares.