According to the OED, the plural of spoonful is today always simply spoonfuls, and indeed, has usually been so historically as well:
- 1625 Laws Stannaries iii. (1808) 17 ― A true note in writing··certifying the just number of pieces, slabs, or spoonfuls of tin above a pound weight.
- 1652 N. Culverwel Lt. Nature ɪ. xv. (1661) 127 ― Babes in Intellectuals must take in··those spoonfuls of Knowledge.
- 1800 tr. Lagrange’s Chem. I. 430 ― Throw this mixture by spoonfuls into a crucible.
The other nouns that end in ‑ful work this way too, even the ones dealing with measurements like armful and quartful, capful and cupful, ladleful and lapful.
That said, there have been exceptions to this in the historical record, including a few within living memory. For example, listed under the β (beta) forms for spoonful, the OED does have some examples of writing its plural spoonsful (in various spellings), but most of these are really quite old:
- 1527 Andrew Brunswyke’s Distyll. Waters D j, ― Dronke of the same water foure spones full at nyght is good agaynste the hote cowgh.
- 1599 A. M. tr. Gabelhouer’s Bk. Physicke 145/2 ― Administre of this water thre spoonesfulle.
- 1863 Bates Nat. Amazon v. (1864) 125 ― We had brought with us a bag of farinha,··and a few spoonsful of salt.
- 1897 Ouida Massarenes xiii, ― Two spoonsful of Cognac in it.
Another historical exception I found in the OED is an irregular boatsful version, from the 17th century:
- 1652 Season Exp. Netherl. 9 ― Loaden by Boats full.
- 1873 Symonds Grk. Poets ix. 289 ― A boatful of careless persons.
- 1883 Contemp. Rev. June 851 ― Whole boatfuls of women.
The OED entry for handful contains an interesting note in its etymology section, which suggests an answer to why things have gone as they have:
Etymology: OE. handfull str. fem., plur. handfulla, f. hand + full adj.: cf. ONor. handfyllr, Ger. handvoll.
Though composed, like mouthful, of sb. and adj., the compound was in OE. and ME. a true sb., inflected as a whole; hence its plural is properly handfuls, not handsful.
Curiously, an irregular plural bucketsful does appear to have some mild currency. The earliest citation almost looks like a place we might today use suspension hyphens:
- 1656 Trapp Comm. Matt. vii. 11 ― He will pour out··as it were by pails or buckets fuls.
- 1843 Spirit of Times 30 Sept. 366/3 ― The rain··came pouring down in buckets-full.
- 1892 C. M. Yonge That Stick II. xxxviii. 181 ― She cried bucketsful.
1936 W. R. Titterton G. K. Chesterton ii. iv. 145 ― Turning out books in buckets-full.
Another counterexample provided is under saucepanful:
- 1976 Horse & Hound 3 Dec. 34/4 ― Two or three saucepansful may be needed simultaneously and it is a good idea to put a spoon in the glass when dispensing.
On the other hand, it is always pursefuls:
- 1846 Mrs. Gore Sk. Eng. Char. (1852) 7 ― The unthrifty, who had flung about pursefulls of those bits of tin, began to hoard the new issue of the mint, as having more significance.