In a small saucepan over low heat, combine milk, cinnamon, nutmeg, and vanilla slowly bringing the mixture to a low boil.
Meanwhile, in a large bowl, whisk the egg yolks with the sugar until the yolks are pale in colour. Slowly add the hot milk mixture to the egg yolks in batches to temper the eggs and whisk until combined.
Return the mixture to the saucepan and cook over medium heat until slightly thick (and coats the back of a spoon) but does not boil. If you’re using a candy thermometer, the mixture should reach 160 °F (70 °C). Add chocolate chips and whisk until melted.
Remove from heat and stir in heavy cream and, if using, spiced rum. Refrigerate until chilled.
When ready to serve, coat the inside of a mug with caramel sauce. Add eggnog to the mug and garnish it with whipped cream and cinnamon.
In a heavy bottom saucepan, whisk together the butter, cream, brown sugar and molasses on medium-high heat.
Let the mixture come to a boil and let it bubble for 2-3 minutes, stirring constantly.
Remove the mixture from the heat and stir in the vanilla. Makes ¾ cup (175 mL).