Preheat the oven to 325F degrees and adjust the oven rack to the middle position. Line a cupcake pan with 4 cupcake liners.
In a small mixing bowl, whisk together the flour, baking powder, salt, and nutmeg.
In a medium mixing bowl, add the sugar and butter. Using a hand held mixer on medium-low speed, beat the butter and sugar until fluffy and the sugar is moistened by the butter. Add the egg yolk and vanilla extract. Continue to beat on medium-low speed until the mixture is light and creamy. Note - Reserve the egg white for the Marshmallow Meringue Frosting!
Sprinkle half of the dry ingredients over the wet ingredients. Beat on low speed until just combined. Pour in the eggnog and continue mixing until just incorporated. Add the remaining dry ingredients and beat until just combined. Do not overmix the batter. It is ok if there are a few lumps.
Pour equal amounts of batter into the prepared cupcake liners. Tip - Use a small cookie scoop to help with portioning and less mess!
Bake for 18 to 22 minutes, or until a toothpick inserted in the middle comes out mostly clean - a few crumbs are ok. Cool in the muffin pan set on a wire cooling rack for 5 minutes. Then transfer the cupcakes to the wire rack to cool completely.