Step 1: Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners. Set aside.
Step 2: In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground nutmeg. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar until light and fluffy, about 2-3 minutes.
Step 4: Beat in the eggs one at a time, incorporating each egg fully before adding the next. Stir in the vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the eggnog. Begin and end with the dry ingredients. Mix until just combined; do not overmix.
Step 6: Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full.
Step 7: Bake for 18-22 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean. Let the cupcakes cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.
Step 8: While the cupcakes cool, prepare the eggnog buttercream frosting. In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar, one cup at a time, mixing until smooth. Beat in 1 tablespoon of eggnog and vanilla extract. Add more eggnog, half a tablespoon at a time, until the frosting reaches your desired consistency.
Step 9: Once the cupcakes are completely cool, frost them using a piping bag or an offset spatula. Garnish with a sprinkle of ground nutmeg if desired. Serve immediately and enjoy!