Cumberland Rum Butter comes from
Cumbria, England. It is extremely simple to create - just warm up some
butter, add lots of
brown sugar and some rum, stir, and allow to cool.
Nutmeg and other spices may be added to improve the taste, although how anything made of rum, sugar and butter can be improved, I don't know.
The end result is traditionally served at Christmas, as it is much too rich to eat all year round. Like Lyle's Golden Syrup and peanut butter it works well with almost every foodstuff, although it is especially useful on chicken or bread. It is not usually eaten neat.
It is not the most healthy food in the world. Note that there isn't anything specifically 'Cumberland' about 'Cumberland Rum Butter'; this node also applies to rum butter in general.