Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
Bars:
In a large, microwave-safe bowl add the white chocolate, butter, and heat to melt, about 2 minutes on high power. Stop to check and stir after 1 minute, and then heat in 20-second bursts, stopping to stir after each burst, until mixture can be stirred smooth. White chocolate is very prone to scorching and seizing up, so watch it carefully and don’t overheat.
Wait momentarily before adding the egg so you don’t scramble it. Add the egg, sugar, eggnog, vanilla, and whisk until combined.
Add the flour, cinnamon, nutmeg, salt, and stir until just combined; don’t overmix.
Turn batter out into prepared pan, smoothing the top lightly with a spatula.
Bake for about 29 to 30 minutes, or until a toothpick inserted in the center comes out mostly clean, or with a few moist crumbs, but no batter; set aside and make the glaze.
Glaze:
In a medium bowl, add confectioners’ sugar, 1/3 cup eggnog, vanilla, and whisk until smooth. As necessary, add additional eggnog (or sugar) to reach desired glaze consistency. Glaze should be fairly thin and easily pourable.
Evenly pour glaze over bars (they don’t have to be cooled completely), smoothing it lightly with a spatula if necessary, but glaze will likely just slide into place.
Sprinkle with nutmeg as desired.
Allow bars to cool in pan uncovered for at least 2 to 3 hours, (or overnight and cover with a sheet of foil) before slicing and serving so glaze can set up.
Notes
Storage: Bars will keep airtight at room temperature for a few days (I’m comfortable storing glazed items at room temp for a few days; then move into the fridge if you have any left), or in the freezer for up to 6 months.Adapted from Lemon Lemonies, Fudgy Banana Bars, Fudgy Pumpkin Bars.