Preheat oven to 325°F (163°C) and position the rack in the center of oven. Place six ramekins in a baking dish that's at least as deep as the ramekins and set aside.
Whisk together the eggnog, heavy cream, ground nutmeg, ground cinnamon, and a pinch of salt in a saucepan, then heat over medium until it just begins to simmer.
In another bowl, whisk together the egg yolks and sugar until the mixture is pale and slightly thickened. Then slowly add 1/2 cup of the warm eggnog mixture to the sugar-egg mixture while stirring constantly. This is to temper the eggs and avoid cooking the eggs. Stir for 30 seconds with a whisk. Repeat until finished.
Pour the mixture through a fine sieve and divide evenly between the ramekins.
Pour hot water into the baking dish until the water comes about 2/3 up the sides of the ramekins. Cover the baking dish loosely with aluminum foil and place in the preheated oven. Bake for about 40-45 minutes or until the custard is set around the edges but slightly jiggly in the center.
Remove the ramekins from the water bath and let them cool to room temperature. Then, cover with plastic wrap once chilled and refrigerate for at least 4 hours or overnight.
Just before serving, sprinkle a thin, even layer of granulated sugar over the custard. Use a kitchen torch to caramelize the sugar until it forms a crispy top. Alternatively, you can place the ramekins under a broiler for a few minutes, but watch carefully to prevent burning.
Allow the sugar to cool and harden for 30 to 60 seconds, serve immediately and enjoy!