The single ingredient is, in fact...
bananas! That's it. Frozen bananas.
Now some people want to get all fancy and start adding other stuff, but then it wouldn't be one ingredient, would it?
Bananas. Preferably a little over ripe. The kind that are starting to turn but not as far along as those used for banana bread.
At first, the banana will look crumbly and start sticking to the sides. You'll need to scrape often.