Line an 8x8-inch baking pan with parchment, leaving overhang handles.
Add sugar, butter, and eggnog to a saucepan.
Heat over medium, stirring until melted and combined.
Bring to a gentle boil and cook, stirring continuously.
Cook until 234–238°F (soft-ball stage), or 8–10 minutes until glossy and thick.
Remove from heat; stir in white chocolate until fully melted.
Add marshmallow crème, vanilla, nutmeg, and cinnamon; mix until silky and uniform.
Scrape mixture into the pan and smooth the top.
Add a light dusting of nutmeg or sprinkles if desired.
Refrigerate 3–4 hours until set.
Lift out by parchment handles and cut into squares with a warm, dry knife.