Lightly grease an 8x8-inch (20x20 cm) square baking pan with butter or cooking spray. Line it with parchment paper or foil, leaving overhang on two sides to act as handles. Lightly grease the parchment or foil as well.

Measure out all ingredients—white chocolate, marshmallow creme, vanilla, spices, rum extract if using, and nuts if using—and keep them close to the stove. Once the sugar mixture reaches temperature, you’ll need to work quickly.

In a medium, heavy-bottomed saucepan (about 3-quart), add the granulated sugar, unsalted butter, fine sea salt, and eggnog. Stir gently over medium heat until the butter is melted and the mixture looks smooth.

Increase the heat to medium-high. Stir constantly as the mixture comes to a full boil with large bubbles across the surface. Attach a candy thermometer to the side of the pan, making sure the tip is in the liquid but not touching the bottom. Continue cooking, stirring often, until the mixture reaches 234–238°F (112–114°C), the soft-ball stage, 5–8 minutes after it begins boiling. If you don’t have a thermometer, drop a small spoonful into very cold water; if you can form it into a soft ball that holds its shape but squishes easily, it’s ready.

Immediately remove the pan from the heat. Stir in the ground nutmeg, ground cinnamon (if using), vanilla extract, and rum extract (if using) until well combined.

Add the white chocolate chips or chopped white chocolate and the marshmallow creme to the hot mixture. Stir vigorously with a sturdy spatula or wooden spoon until the chocolate is fully melted and the fudge is thick, glossy, and uniform with no streaks. Use the residual heat rather than returning the pan to the burner, to avoid scorching.

If using chopped toasted pecans or walnuts, gently fold them into the fudge mixture until evenly distributed.

Quickly pour the hot fudge mixture into the prepared pan. Use a spatula to spread it into an even layer, pressing it gently into the corners and smoothing the top.

While the fudge is still warm, lightly sprinkle extra ground nutmeg and/or cinnamon over the top to mimic a mug of eggnog. Use a light hand so it doesn’t overpower the flavor.

Let the fudge cool at room temperature for 30–45 minutes. Transfer the pan to the refrigerator and chill until firm, at least 2 hours or overnight for easiest slicing.

Use the parchment or foil overhang to lift the fudge from the pan onto a cutting board. With a large, sharp knife, cut into about 1-inch squares, wiping the knife between cuts for clean edges. Serve chilled or at cool room temperature.