Line a 9×5 loaf pan with plastic wrap and set aside.: As you press the plastic into the pan, notice how the smooth surface gleams and creates an easy release when the cake is set. This wrapping prevents the filling from sticking and helps you lift the whole cake out in one piece. Make sure the wrap extends over the edges so you can cover the top too. A frequent oversight is using too little wrap which makes unmolding tricky and may tear the sides, so be generous when lining.
Using a hand mixer or stand mixer with the whisk attachment, whip the cream cheese until smooth.: The cream cheese should transform from slightly rigid to soft and glossy, and you will see it lighten in color as air is incorporated. Beat on medium speed, pausing to scrape the bowl so there are no stubborn lumps. If the cream cheese smells tangy and fresh, it is good to use; if overly sour discard it. Avoid under whipping which leaves bits of solid cream cheese in the filling.
Add in the Instant vanilla pudding, eggnog, and nutmeg and mix on low until fully combined. I found that sometimes hand mixers are too fast for this step so I like to combine them with a whisk or use a very slow mixer.: When these ingredients meet, the mixture becomes fragrant with sweet vanilla and warm nutmeg . Mix gently on low so the pudding hydrates and the eggnog integrates without splattering. The batter should be smooth and slightly fluffy, not runny. If your mixer is too fast the pudding may clump, so slow down or mix by hand as needed to avoid lumps.
Fold in the Cool Whip gently with a spatula.: As you fold, watch the mix lighten and become billowy; it should hold soft peaks and emit a cool, creamy aroma. Use wide strokes, turning the bowl and scraping the sides so the Cool Whip is evenly distributed. Overfolding will deflate the air and yield a denser final texture, so stop as soon as uniform ribbons form.
Layer the bottom of the prepared pan with cinnamon graham crackers and top with 1/3 of the eggnog mixture.: Arrange a single, neat layer of cinnamon graham crackers so they cover the pan base, then spoon a third of the filling across them. Press lightly so the filling adheres and you get even contact that helps the crackers soften just enough. You want a balance between wet and dry, if you press too hard the crackers may break and the layers will become muddled.
Repeat the graham cracker and eggnog layers 2 more times. You should have 3 layers of crackers and 3 layers of eggnog mixture.: Building uniform layers ensures attractive slices and consistent texture. By the final layer you should see three tiers of crackers and three of filling. Keep the distribution even so slices hold their shape. A common mistake is piling more filling in one zone which leads to uneven freezing and sagging when sliced.
Cover the pan with plastic wrap and freeze for at least 6 hours.: As the cake chills, the filling firms and the crackers soften just enough to become tender yet retaining a pleasant structure. In the freezer you may notice faint icing crystals on the surface if the wrap is not sealed completely, so press the wrap snugly against the filling. Avoid skipping sufficient freezing time, as serving too soon yields a sloppy, unset slice.
When you're ready to serve simply remove it from the freezer, pull it out of the pan, remove the plastic wrap, slice and serve frozen.: Let the loaf rest at room temperature for 2 to 3 minutes before slicing so the knife glides smoothly; the edges should be clean and the filling should hold. Use a long, sharp knife and wipe between cuts for perfect presentation. Cutting while rock hard can chip the slice, while cutting after too long may make it lose shape.
Optional, top the icebox cake with whipped cream or nutmeg.: A final dusting of freshly grated nutmeg or a dollop of whipped cream adds visual polish and a burst of aroma. If you add whipped cream, pipe it on quickly so it doesn’t weep as the cake warms slightly. Overdoing the topping can overshadow the balanced flavors of the filling, so use it sparingly.