I have this term in an 18th-c source, but I'm wondering if it is used currently. I think that it is equivalent to what is (or rather was) called "a pat of butter" (about a pound traditionally, but in modern usage about two tablespoons). The dictionnary of this site gives "pat of butter" as motte de beurre, which seems close to me. The 18th-century French dictionaries typically define pain de beurre as une certaine quantité de beurre. Not much help... I do remember that in Little Red Riding Hood's basket there was "a pat of butter." As a child I said to myself, "what is that?" and years later saw Perreault has un pot de beurre. I just may drop into a local fromagerie to ask, but if anyone has any ideas they would be welcome!