Preheat oven to 350ºF degrees. Line a 12-count muffin pan with cupcake liners.

In a mixing bowl, whisk together the flour, baking powder, baking soda, spices, and salt until combined; set aside.

In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium speed until lightened and creamy. Beat in the eggs, one at a time, until combined, scraping down the sides and bottom of the bowl as needed. Beat in the Greek yogurt and vanilla on medium speed.

Pour the dry ingredients into the wet ingredients and mix until combined. Add the eggnog, then mix until just combined, scraping down the sides and bottom of the bowl as needed.

Fill cupcake liners 2/3 of the way full. Bake for 19-22 minutes or until a toothpick inserted in the center comes out clean or with few crumbs attached. Allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

In a small saucepan over medium heat, whisk flour and eggnog together, stirring constantly, until it thickens. It should look like thick pancake batter. Remove from heat and immediately pour milk mixture into a bowl to cool completely.*

After the milk mixture has come to room temperature, use a stand mixer fitted with the whisk attachment or a beater to cream the butter, brown sugar, vanilla, and salt together on medium high until light and fluffy, about 3 minutes.

Add the completely cooled eggnog mixture, and beat it for about 7-10 minutes on high until it looks like whipped cream. The mixture will look curdled at first, but as you continue whipping it, it should come together. If you can still see streaks of butter, keep beating it until you can’t.**

Frost or pipe buttercream onto cupcakes, as desired. Garnish with sprinkles.