Wash fiddleheads in a bowl of water, rubbing lightly to release the papery brown coating. Pour off the water and papery part, repeat until clear.
Trim off just a thin slice at the end of each cut stem - the dry and brownish part. Pull off any papery bits that are still curled up in the fronds - you probably won't get them all.
Bring a pot of water to boil. Place fiddleheads in a steamer insert, and place over the water. Cover and steam for 10 minutes.
Optional: Melt butter in a fry pan, and briefly saute fiddleheads in it, until well covered in butter, and a few spots lightly brown.
Serve at once.
Otherwise, simply serve hot with butter, if desired.