Prep the pan: Heat oven to 325°F (163°C). Line a 9x9-inch baking pan with parchment, leaving overhang for easy lifting.
Make the crust: Stir cookie crumbs, melted butter, sugar, and salt until the texture resembles wet sand.
Press firmly into an even layer in the pan.
Par-bake: Bake the crust for 8–10 minutes until set and fragrant. Let it cool slightly while you make the filling.
Beat the cream cheese: Using a mixer on medium-low, beat cream cheese until smooth and creamy, 1–2 minutes. Scrape the bowl well.
Add sugar and starch: Mix in sugar, flour (or cornstarch), and salt until combined.
Avoid overbeating to limit air bubbles.
Finish the filling: Add eggs one at a time on low. Mix in eggnog, vanilla, nutmeg, and cinnamon just until smooth.
Pour and smooth: Pour filling over the warm crust. Tap the pan gently on the counter to release bubbles.
Bake: Bake 28–35 minutes, until edges are set and the center jiggles slightly like gelatin.
Do not overbake.
Cool gradually: Turn the oven off, crack the door, and let bars rest 10 minutes. Transfer to a rack to cool to room temp.
Chill: Refrigerate at least 4 hours, preferably overnight, until fully set.
Slice and serve: Lift out with parchment, cut into bars with a sharp knife wiped clean between cuts. Top with whipped cream and a sprinkle of nutmeg if you like.