Prepare the Cupcake Batter:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Set aside.
3. Cream the Butter and Sugar: In a large bowl, cream together the softened butter and sugar using an electric mixer on medium speed until light and fluffy, about 3-4 minutes.
4. Add Eggs: Beat in the eggs one at a time, making sure each egg is fully incorporated before adding the next.
5. Add Wet Ingredients: Stir in the eggnog, vanilla extract, and rum extract (if using) until just combined.
6. Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet mixture, mixing until smooth and well-combined. Be careful not to overmix.

Bake the Cupcakes:

7. Fill the Cupcake Liners: Divide the cupcake batter evenly among the cupcake liners, filling each about two-thirds full.
8. Bake: Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.

Prepare the Buttercream Frosting:

9. Cream the Butter: In a large bowl, beat the softened butter using an electric mixer until smooth and creamy, about 3 minutes.
10. Add Powdered Sugar: Gradually add the powdered sugar, 1 cup at a time, beating well after each addition.
11. Add Eggnog and Spices: Stir in the eggnog, vanilla extract, cinnamon, nutmeg, and salt. Beat until the frosting is light and fluffy, about 3-4 minutes.

Frost the Cupcakes:

12. Frost the Cupcakes: Once the cupcakes are completely cool, spread or pipe the buttercream frosting on top of each cupcake.
13. Garnish: Sprinkle with a pinch of ground cinnamon and, if desired, top with crushed candy canes for a festive touch.