Make the Chili Mayonnaise: in a small bowl, whisk the mayonnaise ingredients together; cover and refrigerate until ready to use.
Make the burgers: preheat oven to 425°; scrub the sweet potato, pat dry, and poke 5-6 times with a fork.
Bake, about 1 hour, until tender; let cool for 15 minutes, until it can be handled easily.
Remove the skin and gently crumble the flesh into a large bowl.
While the potato is baking, heat the olive oil in a large pot over medium heat.
Add in the bell pepper, garlic, onion, tomato, cumin, pepper flakes, cardamom, and bay leaf; cook about 8-10 minutes, until the peppers are soft, the onion is translucent, and the tomatoes release their juices.
Stir in the black beans; set aside to cool.
Drain juices from pan and reserve; remove bay leaf.
Preheat a broiler or grill to medium heat.
Add the pepper-bean mixture to the sweet potatoes; season to taste with salt and pepper.
Using you hands, mix until the ingredients are evenly incorporated.
Form 4 patties of equal size and lightly brush them with olive oil.
Broil or grill burgers, flipping once, until they are crisp on the outside, about 4 minutes on each side.
Serve on multi-grain buns topped with Chili Mayonnaise.