Line a square pan with foil and grease it well to prevent sticking.

In a saucepan over medium-high heat, combine the unsalted butter, granulated sugar, and full-fat eggnog.

Stir often as it comes to a boil, ensuring it reaches exactly 234°F using a candy thermometer.

Remove the saucepan from heat and quickly stir in the white chocolate chips until they’re fully melted and smooth.

Fold in the marshmallow fluff, pure vanilla extract, and ground nutmeg until everything is thick and evenly blended.

Pour the fudge mixture into the prepared pan, smoothing the top with a spatula.

Let the fudge cool at room temperature until it's firm, then cut into squares and serve.