Separate the egg yolks and whites into two mixing bowls. Set egg whites aside in the fridge.
Whisk the egg yolks, brown sugar and salt together until the egg yolks are fluffy and light in colour.
Add all the milk, nutmeg and cinnamon to a medium sized pot over medium low heat. Cook until it starts to steam and just begins to boil. Then remove from heat and use to temper the egg yolk mixture.
While the mixer is running or while whisking, slowly pour a stream of the hot milk into the egg yolks to temper them. Pour the mixture back into the pot with the rest of the milk.
Place the pot back over a medium low heat and stir occasionally until they reach a temperature of 160 F. The eggnog should be slightly thickened so that it coats the back of a spoon. Remove from heat and let cool slightly while mixing egg whites.
With a stand mixer or handheld mixer beat the egg whites and sugar until soft peaks form not stiff (instructions to make egg whites safe listed in the post instructions above recipe).***
After the egg yolk and milk mixture have cooled down some, fold/stir in the fluffy egg whites and the vanilla. Place in the fridge to chill for at least 4 hours before serving.