Heat the Oven & Prep Your Pan: Heat the oven to 350°F. Place the cupcake liners in the cupcake tin.
Sift the Dry Ingredients: In a large bowl, sift the flour, sugar, baking soda, baking powder, salt, nutmeg, and cinnamon.
Whisk the Wet Ingredients: In a medium bowl, whisk together the egg, egg whites, oil, vanilla extract, rum extract, and sour cream. A clumpy batter is okay!
Add Wet to Dry: Add the wet ingredients to the dry ingredients and stir until just barely combined (you'll still see streaks of flour). Slowly add in the eggnog, and stir until just combined. Be careful not to over-mix the batter!
Fill the Cupcake Liners & Bake: Fill the cupcake liners evenly until they are 2/3 full. Bake for 18 minutes until the cupcakes are springy to the touch. If you are unsure, test with a toothpick. It should come out dry. When the cupcakes are done, allow them to cool slightly before removing them from the tins. The cupcakes should finish cooling on a wire rack.