How to make rice crispy treats with no marshmallows, eggs or gelaten
Aquafaba

Aquafaba (/ˌɑːkwəˈfɑːbə/) is the name for the viscous water in which legume seeds such as chickpeas have been cooked.
Due to its ability to mimic functional properties of egg whites in cooking, aquafaba can be used as a direct replacement for them in some cases, including meringues and marshmallows. Its composition makes it especially suitable for use by people with dietary, ethical, or religious reasons to avoid eggs.1

Aqua Faba features heavily in Vegan cookbooks. You can get it by capturing the liquid when you drain a can of chickpeas.
Here's a recipe that might get you started.