I would definitely say, and write, 'spoonfuls'. I think to do otherwise is simply confusing what is being pluralised - the referent is the 'spoonful', ie, the contents, not the (singular) spoon.
spoonsful because (1.) I can be quite pedantic on occasion and 2.) while I say spoonsful, I am thinking "spoons full" and (3.) In earlier days I wanted my children to be able to spell well and pronunciation is a big part of that skill.
Well, actually, it probably depends on whether you regard the word "spoonful" as a word in its own right - a unit of measure - or if you see it as two separate words run together for a particular purpose within some context or other.
I happen to see it as a measure. A spoonful is an amount. Two spoonfuls is twice the amount. I therefore happen to think that spoonsful is just wrong.
Perhaps you do things differently over there, but if I'm measuring out sugar or whatever, I usually use the same spoon three times rather than three different spoons once each. So it's definitely spoonfuls -- the amount is pluralised rather than the spoons.
OK, Bingley, that's an excellent point. Makes sense to me!You have converted me to spoonfuls. On my fridge I have two large, magnetic metal "buttons" which give the dry and liquid measure conversions. Very handy when cooking. Now what I really need are two conversion "buttons" for European to American measures. It was quite an adventure to figure out the directions for my O'Douls Irish Bran Bread mix. But I did it and now have lovely bread for breakfast every morning. Yum!