I usually make about a pint of this at a time and keep it in the fridge; when you want an after-dinner beverage, just scoop some into a mug with hot water and rum.
In a bowl, cream together butter, sugar, cinnamon, nutmeg, cloves + salt. Refrigerate until almost firm. Spoon 2 Tbs of butter mixture into 12 small mugs. Pour about 3oz of rum into each mug (filling ~halfway). Top with boiling water.