In a saucepan over medium-low heat, combine the first four ingredients, stirring until combined. Let simmer for about 10-12 minutes, stirring consistently, until mixture is warm.
If you start to see bubbles around the side of the saucepan, continue to stir. You don't want the mixture to reach a rolling boil.
When mixture is heated through, remove from heat and add cinnamon, stirring to combine.
Garnish each glass with fresh grated eggnog, if desired.
Recipe by In Good Taste at http://ingoodtaste.kitchen/spirited-cinnamon-eggnog/