Preheat the oven to 350ºF (180ºC). Lightly grease a standard 12-cup muffin tin with melted butter and line each cavity with a paper muffin wrapper. Set aside.
Whisk together the flour, baking powder, cinnamon, nutmeg, and salt together in a mixing bowl until well combined. Set aside.
Place the butter and sugar in a second mixing bowl. Use a hand or stand mixer to cream the two together until light and fluffy, about 1 to 2 minutes.
Add the eggs and vanilla and mix well on medium-high speed to combine. Add the eggnog to the bowl and again, mix well.
Use a fine mesh sieve to sift the flour into the butter-eggnog mixture. Once the flour has been sifted, there may be some bits of nutmeg and salt that didn’t pass through the sieve. Dump them into the bowl.
Switch to a rubber spatula or wooden spoon. Gently fold and stir the cupcake batter together until the flour is just mixed in. Divide the batter evenly between the 12 muffin cups in the prepared tin and bake for 20 to 22 minutes, or until the edges of the cupcakes are pale gold and the tops spring back when pressed gently with a fingertip. Transfer the muffin tin to a wire rack to cool for 10 minutes. Remove the cupcakes from the tin and set on the rack to cool completely before frosting.
Make The Eggnog Buttercream Frosting
Place the butter in a mixing bowl. Use a hand or stand mixer to cream the butter until light and fluffy, about 1 to 2 minutes.
Use a fine mesh sieve to sift the powdered sugar into the bowl. Mix on low speed to combine.
Add the rum extract and continue to mix.
Add eggnog by the teaspoon, mixing in between each addition, until you reach your desired consistency. You will want the frosting to be a bit stiff if you intend on piping the frosting. Once you are happy with the frosting, use it to decorate the fully cooled eggnog cupcakes.