To make the butter, vigorously mix ingredients together those ingredients in a medium bowl. Set aside until ready to use. (You can make in advance and store in the refrigerator for three days.)
Stir together rum and water.
Place 1 tablespoon butter each into 2 warmed cups, then pour equal parts of the drink over the top. (If you would like it a little sweeter, feel free to add a touch more sugar.)