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 the saucepan into the ice bath and stir as the eggnog cools to room temperature.
Fold in 1 1/2 cups of the whipped cream into the custard. Chill the remaining whipped cream for serving.
Divide the eggnog mousse into individual ramekins or serving cups. Cover each cup with plastic wrap and store in the refrigerator until ready to serve. The mousse can be prepared up to two days in advance.
When ready to serve, sweeten the remaining whipped cream with a tablespoon of confectioners sugar, whisking well to combine. Place a dollop of whipped cream on each serving of mousse, and garnish with a sprinkling of cinnamon. Enjoy immediately.
Notes
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.