Using a mixing bowl and whisk attachment that has been chilled in the freezer, whip the heavy cream until stiff peaks form. Store in a tightly sealed container in the refrigerator until needed.
In a small bowl, place cold water and sprinkle the gelatin. Let it stand for about 10 minutes, until the gelatin is soft.
Prepare an ice bath and set it aside.
In a medium saucepan, heat the milk, 1/4 cup of the sugar and vanilla over medium low heat, whisking periodically until the sugar is fully dissolved and the milk begins to steam and slowly comes to a gentle simmer. As the milk heats up, whisk the remaining sugar into the egg yolks, beating continually until the eggs are paler in color and smooth in texture.
When the milk is just scalded, pour about 1/4 cup of the milk in a gentle stream into the egg yolks as you whisk it together. Add another 1/4 cup of milk and whisk thoroughly. Pour the warmed egg yolks into the saucepan and whisk into the rest of the milk. Add the cinnamon and nutmeg and stir constantly over medium low heat until it begins to thicken, about 3-5 minutes.
Remove the custard from the heat and stir in the softened gelatin until it dissolves.
Place saucepan into the ice bath and stir as the eggnog cools to room temperature.
Make it ahead: Prepare the eggnog mousse in advance of your holiday festivities and leave it in the refrigerator for up to 2-3 days.