Found in old Swedish lore, witch's butter is not an example of delicious Scandanavian food - it refers to the the vomit from the supposed witches' cat. Centuries ago it was thought that the felines owned by them were a gift from the Devil, so when they were seen wandering they attracted suspicion and superstition - the Swedes thought that the witches would send their cats out into the neighbourhood to steal food. The cats would invariably eat too much, so the tubby moggies would then vomit into the ailurophobic neighbours yard, which was thought to curse the house.
Witches' butter is also a kind of freshwater algae, Nostoc. It is also called 'star spittle' because summer rain 'magically' causes the algae to swell to a gelatinous blob, with the sudden appearance making it seem to have fallen from the sky.
If someone offers you a dish of witch's butter it would be best to decline....neither is an appetising option.