In a bowl, cream together the butter, sugar, cinnamon, nutmeg, cloves, and salt.
Refrigerate until almost firm. Spoon about 2 tablespoons of the butter mixture into 12 small mugs.
Pour about 3 ounces of rum into each mug (filling about halfway).
Top with boiling water (to fill the remaining half), stir well, and serve immediately.
Serves 10 to 12