1. Preheat oven to 350°F (176°C) and line a cookie sheet with a silicone baking mat or parchment paper.
2. Cream the butter and brown sugar together for 3-4 minutes, until light and fluffy. Don’t skimp on the creaming time.
3. Add the egg and molasses and mix until well combined.
4. Combine the dry ingredients in a separate bowl, then add to the wet ingredients and mix until smooth. Dough will be thick.
5. Make balls of cookie that are 1 1/2 tablespoons in size and place them on the prepared cookie sheet.
6. Bake for 7-8 minutes or until the centers of the cookies are just cooked. Don’t over bake.
7. Allow cookies to cool for 3-4 minutes, then remove to a cooling rack to cool completely.
8. To make the eggnog icing, combine all the ingredients except for the powdered sugar in a small sauce pan. Heat on low until the sugar is melted, then turn heat to medium to bring to a boil.
9. Boil for 3 minutes, stirring occasionally, then remove from heat and immediately pour into a heat-proof glass bowl.
10. Whisk in powdered sugar and allow to cool slightly and thicken. If you find your icing to be a little thick, just add a touch more water to thin it back out. If it cools too much and firms before adding to the cookies, just microwave it for about 10 seconds.
11. Frost the cookies with the icing and add sprinkles, if desired.
12. Store in an airtight container until ready to serve.