In a medium saucepan over medium heat, combine the granulated sugar, butter, and sweetened condensed milk. Stir continuously until the mixture is smooth and the sugar is dissolved.

Once the mixture is smooth, add the eggnog, vanilla extract, nutmeg, and cinnamon. Stir to combine.

Bring the mixture to a gentle boil, then reduce the heat and simmer for about 5 minutes, stirring frequently.

Remove from heat and stir in the white chocolate chips until fully melted and smooth.

Line an 8x8 inch square baking pan with parchment paper, leaving some overhang for easy removal.

Pour the fudge mixture into the prepared pan and spread evenly.

Sprinkle the chopped pecans (or walnuts) on top and gently press them into the fudge.

Let the fudge cool at room temperature for about 1 hour, then refrigerate for at least 2 hours until set.

Once set, remove the fudge from the pan using the parchment overhang and place on a cutting board.

Drizzle with melted white chocolate and sprinkle with additional ground cinnamon for garnish, if desired.

Cut into squares and serve. Store leftovers in an airtight container in the refrigerator.