Preheat the oven to 350°F and line a muffin pan with cupcake liners.
In a stand mixer, beat butter and brown sugar on medium speed using a paddle attachment until light and creamy, about 2 minutes.
1/2 cup salted butter, 1/2 cup light brown sugar
Add the egg, milk, molasses, and vanilla, mixing well and scraping down the sides and bottom of the bowl with a rubber spatula. Don't worry if the ingredients don't totally combine - they will finish coming together with the addition of the dry ingredients in the next step.
1 large egg, 1/2 cup whole milk, 1/2 cup molasses, 1 teaspoon vanilla extract
Add the flour, baking powder, baking soda, salt, ginger, cinnamon, nutmeg, and allspice, mixing on low speed just until combined and making to sure scrape the bottom and sides of the bowl again.
1 1/3 cups all-purpose flour, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, 1 1/2 teaspoons ground ginger, 1 teaspoon ground cinnamon, 1/2 teaspoon ground nutmeg, Pinch of ground allspice
Fill cupcake liners 2/3 full. Bake for 19-21 minutes, until a toothpick comes out with just a few crumbs when inserted into the center of a cupcake, then cool completely before frosting.
1 batch eggnog buttercream frosting or other favorite frosting recipe