In a small bowl, combine the gelatin powder and cold water. Allow it to bloom for about 5 minutes.
In a medium saucepan, whisk together the egg yolks and granulated sugar until creamy.
Add the whole milk, vanilla extract, nutmeg, and cinnamon to the saucepan and mix well. Cook over medium heat while constantly stirring until it thickens, about 5-7 minutes (do not let it boil).
Remove from heat, then stir in the bloomed gelatin until fully dissolved. Allow the mixture to cool to room temperature while occasionally stirring.
In a large mixing bowl, whip the heavy cream until stiff peaks form.
Gently fold one-third of the whipped cream into the cooled eggnog mixture to lighten it, then gradually fold in the remaining whipped cream until just combined.
Spoon the mousse into individual serving dishes or a large bowl.
Refrigerate for at least 2 hours or until set.
Notes
Garnish each serving with whipped cream and a sprinkle of nutmeg for an elegant touch.