The one secret ingredient is . . .
First, cut the bananas into chunks. You don't want to freeze whole bananas, or even halves as they are too big to blend well. Chunks don't have to be small, I'd say about 1 inch (2.5 cm) square. Its easier to cut them before freezing, but its not impossible to cut frozen bananas, just be careful! Add chunked frozen bananas to a blender and puree until you have amazing softserve.
I find living in Utah that I have to add about 1 Tbsp cold water per banana because they dry out a bit in the freezer so that is why single ingredient is in quotes! It takes two ingredients when I make it :)
This is famous as a raw dessert, but that doesn't stop me from drizzling non-raw chocolate syrup on it, but there are some good raw chocolate syrup recipes out there, its just not my thing . . .