Spoonsful is correct but it makes you sound like a twat. Spoonfuls is great. Just like bucketfuls and lots of others. English is always wot people speak. It's a good guide.

I am English and I would say 'spoonsful' and 'bucketsful'. 'Spoonfuls' sounds really ugly .... and I dare you to make your comments to my face. ;-)

I agree that '-ful' is not needed and 'spoons' and 'buckets' would do but it's nice to have the option. English is a rich and varied language.

DC