01 - Line an 8x8-inch baking pan with parchment paper, ensuring there is enough overhang to lift the fudge out after chilling.
02 - In a medium saucepan over medium heat, combine unsalted butter, eggnog, and granulated sugar. Stir frequently and bring the mixture to a gentle boil.
03 - Reduce the heat to low and simmer for 4 to 5 minutes, stirring constantly to avoid scorching.
04 - Take the saucepan off the heat. Add white chocolate chips, ground nutmeg, ground cinnamon, and vanilla extract. Stir until the chocolate is fully melted and the mixture is smooth.
05 - Immediately pour the warm fudge mixture into the prepared pan, smoothing the surface evenly with a spatula.
06 - If using, sprinkle freshly grated nutmeg evenly over the top.
07 - Let the fudge cool at room temperature for 30 minutes, then transfer to the refrigerator and chill for at least 2 hours until firm.
08 - Lift the fudge from the pan using the parchment paper overhang, then cut into 36 squares with a sharp knife.