Prepare the Pan:

Line a 9x9-inch baking dish with parchment paper or aluminum foil, allowing the edges to overhang for easy removal. Grease lightly with butter or cooking spray.

Melt Sugar and Butter:

In a medium saucepan, combine the granulated sugar, butter, milk, and salt. Cook over medium heat, stirring constantly, until the mixture comes to a boil.

Cook to the Right Temperature:

Once boiling, attach a candy thermometer to the pan and cook the mixture to 234°F (112°C), stirring frequently. This is the “soft-ball stage.” If you don’t have a thermometer, drop a bit of the mixture into cold water—it should form a soft, pliable ball.

Add Flavorings and Chocolate:

Remove the pan from heat and quickly stir in the vanilla extract, semisweet and milk chocolate chips, and marshmallow creme. Stir vigorously until the mixture is smooth and glossy.

Pour into Pan:

Immediately pour the fudge into the prepared pan and spread evenly with a spatula.

Cool and Set:

Let the fudge cool at room temperature for about 30 minutes, then transfer to the refrigerator for at least 2 hours, or until fully set.

Cut and Serve:

Once set, lift the fudge out of the pan using the parchment overhang. Cut into squares and serve.