Pour 2 tbsp (30 mL) egg nog into a small dish. Set aside. In another small dish, mix together 1 tbsp (15 mL) sugar and 1/2 tsp (2 mL) cinnamon. Dip each mug rim first into the eggnog then into the sugar mixture. Set aside.
Heat remaining egg nog and cinnamon in a heavy saucepan over medium heat, until hot but not simmering. Whisk until frothy. Set aside. Divide coffee among 8 mugs. Top up with egg nog and foam before serving.
Add 1 oz (30 mL) spiced rum or coffee- or chocolate-flavoured liqueurs to each mug for extra warmth.