Add ice to a mule mug. Add cinnamon whiskey, lime juice and the spent lime shell (if you used fresh).
2 ounces cinnamon whiskey, ¾ ounce lime juice
Add the apple cider and ginger beer.
2 ounces apple cider, 3-4 ounces ginger beer
Garnish with an apple slice and a cinnamon stick.
1 slice apple, 1 stick cinnamon
Notes
If you'd prefer not to use Fireball, you can try spiced rum for a similar cozy flavor. Applejack, brandy, bourbon or vodka would all work fine as well.