Ingredients: rum, brown sugar, boiling water.
Put a teaspoon (or more) brown sugar into a mug or teacup, a shot of rum (light or dark), and fill with boiling water.
Float a pat of butter on top to make a Hot Buttered Rum (which ought not be consumed on a full stomach). Try adding a splash of a favourite liqueur or using coconut-flavoured rum (eg Malibu). Brown sugar can be replaced with white, and gin with whiskey to make a Whiskey Toddy or Whiskey Skin.