Line an 8x8 or 9x9-inch pan with foil and grease well.
Combine butter, eggnog, cream, and sugar in a saucepan. Bring to a boil over medium heat, stirring occasionally.
Cook until a candy thermometer reads 234–235°F (soft-ball stage), about 8–10 minutes.
Remove from heat and stir in white chocolate chips until melted and smooth.
Stir in marshmallow creme, nutmeg, and vanilla until combined.
Pour into the prepared pan, smooth the top, and tap out air bubbles. Sprinkle with extra nutmeg.
Cool completely at room temperature, then refrigerate at least 2 hours before cutting into squares.
