Bananas work great as an egg replacer due to their natural goopyness.  Keep in mind that whatever you put bananas in will undoubtedly turn into banana-whatever-the-name-of-your-recipe-is. This can be great for things like vegan cakes, pancakes and sweet breads but awkward in things where you're not going for the whole banana thing. Banana's aren't as powerful binders as Egg Replacer Powder or Flax Seed Egg Replacer but they're more powerful than using yogurt.

An egg contains about ¼ cup of liquid so using banana purée as a substitute in a recipe consists of:

¼ cup banana purée.