Makes an unfeasible quantity of divine badness.
8oz unsalted butter
6oz soft light brown sugar
4-5 tbsp rum
Cream the butter until pale and then beat in the sugar until it's all light and fluffy. Don't try doing this by hand unless you are a masochist.
Add the rum a teeny dribble at a time while beating furiously. Too much and it will curdle.
Spoon into an attractive bowl and keep in the fridge for dolloping over Christmas Pud, mince pies, or just eating out of the bowl.