The classic legume-based dessert are black bean brownies, which have been a staple of gluten-free desserts for a long time, in part because they are so easy to make: instead of having to buy fancy gluten-free flours, one only needs to puree a can of black beans (usually available for <$2 in Canadian supermarkets). However, traditionally black bean brownies depend on a lot of eggs to hold them together. I rarely buy eggs and so I try to find recipes that don't use them, both for allergy reasons and also just convenience, but it's taken me a number of years to track down a vegan black bean brownie recipe that works. But never fear - I have succeeded!
Shortly after, I was introduced to a recipe for peanut butter-chocolate chip squares. They are sweetened with maple syrup, and thus are fairly low sugar (unless you buy overly-sweetened chocolate chips, like I did by mistake). But definitely an awesome high-protein snack when the sweet tooth kicks in.