In a large bowl, cream together the butter and sugar until light and fluffy.
Add the egg to the butter and sugar and whip until smooth. Pour in the eggnog and vanilla and blend until a smooth batter forms.
Sift the Dry Ingredients:
In a second bowl, sift together all the remaining dry ingredients and set aside.
Finish the Cupcake Batter:
Fold in the bowl of dry ingredients and mix for 1-2 minutes until just combined and smooth. Do not over-mix the batter (otherwise the cupcakes will fall in while baking).
Pour the batter into a lined cupcake tins, filling each cupcake liner ⅔ full with batter.
Bake the Eggnog Cupcakes:
Transfer to the oven for 20-25 minutes or until an inserted toothpick comes out clean. Remove and let cool completely.
Prepare the Eggnog Buttercream:
In a bowl, mix together the eggnog, butter, cinnamon, nutmeg, and vanilla.
Sift the powdered sugar. Add half the sifted powdered sugar to the butter and mix with a hand mixer until mostly combined. Add the rest of the powdered sugar and continue mixing until smooth and creamy.
Frost the Cupcakes:
Once the cupcakes have cooled, transfer the buttercream to a piping bag and frost the cupcakes. Add your favorite festive sprinkles and enjoy!