Prep the pan: Line a 9x13-inch baking pan with parchment, leaving overhang for easy lifting.
Lightly grease the sides.
Heat the oven: Preheat to 350°F (175°C).
Make the crust: In a bowl, combine cookie crumbs, sugar, nutmeg, cinnamon, and salt. Stir in melted butter until the mixture looks like damp sand.
Press and pre-bake: Press the crust firmly and evenly into the pan. Bake for 8–10 minutes until fragrant and lightly set.
Cool for 5 minutes.
Beat the cream cheese: In a large mixing bowl, beat softened cream cheese on medium speed until completely smooth and fluffy, about 2 minutes. Scrape the bowl.
Add sugars: Beat in granulated and brown sugars until silky and no lumps remain, 1–2 minutes. Scrape the bowl again.
Mix the wet ingredients: Add vanilla, nutmeg, cinnamon, and salt.
Beat until combined. Mix in the cornstarch until no streaks remain.
Incorporate eggnog and eggs: Beat in eggs one at a time on low speed. Add eggnog and mix just until smooth. Do not overmix; you want minimal air bubbles.
Pour and smooth: Pour the filling over the warm crust.
Tap the pan gently on the counter to release air bubbles.
Bake: Reduce oven to 325°F (165°C). Bake for 30–38 minutes. The edges should be set and the center slightly wobbly, like jello.
Cool gradually: Turn off the oven, crack the door, and let the bars rest for 10 minutes.
Remove and cool at room temperature for 1 hour.
Chill: Refrigerate at least 4 hours, preferably overnight, until fully set.
Slice: Lift out using the parchment. Warm a sharp knife under hot water, wipe dry, and cut into neat bars. Clean the knife between cuts for sharp edges.
Finish: Top with a dollop of whipped cream and a light dusting of nutmeg if you like.