Again from our trusty Old Mr. Boston book. I first tasted this at my daughter's home and then had to go buy some spiced rum so I could make it for myself.
2 ounces spiced rum
3/4 ounce triple sec (I use Cointreau)
3/4 ounce lemon juice
1/2 ounce simple syrup
Combine all in a cocktail shaker, add ice, and shake. Pour into a cocktail glass.
(You are supposed to run a lemon wedge round the rim of your glass and dip it in cinnamon sugar before pouring the cocktail in ... but that's not how I roll ... too sweet.)