For the butter:
1 pound light brown sugar
16 tablespoons (2 sticks) unsalted butter, at room temperature
2 teaspoons cinnamon
2 teaspoons freshly grated nutmeg
1 to 2 teaspoon allspice
2 teaspoons vanilla extract
For each drink
1 1/2 ounces rum, preferably Mount Gay Eclipse
Boiling water, as needed
For the butter: Beat together the brown sugar, butter, spices and vanilla extract until well combined. Refrigerate in an airtight resealable container until ready to use.
For each drink: Combine 2 heaping tablespoons of the batter and the rum in a warmed coffee mug. Add boiling water to fill to the top, and mix well. Serve with a spoon.