To make the eggnog base, pour the milk and cream in to a saucepan. Add the spices (including the empty vanilla pod) and place on a medium heat. Bring to a simmer (but not boiling) and allow the hot milk mixture to cool for 5 minutes to allow the flavours to infuse.
Meanwhile place the egg yolks and sugar in a large mixing bowl. Whisk well until light and thickened. Strain the warm milk into a jug through a sieve. Pour the spiced milk into the egg mixture in a steady stream whisking well. Stir in the rum.
You can either place the mixture in a cooled sterilised bottle with a tight fitting lid and allow the flavours to develop in the fridge.
If you decide to do this you can freeze the egg whites in a sandwich bag until you’re ready to use. Then defrost them in the fridge before whisking.
If you would like to serve straight away, place the egg whites in a separate bowl and whisk the whites until soft peaks form. Adding the whisked whites will make the eggnog thicker and frothier.
Mix the demerara sugar and ground cinnamon on a plate. Have a small bowl of rum or water ready.
Fold the egg whites into the eggnog mix then pour into a serving jug. Dip the rims of serving glasses into the rum or water and then dip straight into the sugar and cinnamon mix. Divide the eggnog between glasses and finish with a grating of nutmeg or cinnamon for garnish.