They are basically lumps of butter and salt held together with a trace amount of flour. The versions available from mainstream stores (e.g. M&S) or from recipes online are fairly tame - for the real effect they need to be purchased from a small local baker in some remote fishing village.
Best butteries I've ever had were actually on a trawler in the North Sea - where they were served hot awash in molten salted butter.
You'd be hard pushed to find a buttery actually made of butter these days. Even the local bakers are using vegetable oil and fat. Still, they are beautiful, flaky rolls of salty tastiness.
