In a large pot, bring water to a boil and add the cubed sweet potatoes. Cook for about 15 minutes or until tender. Drain and let cool slightly.
In a large mixing bowl, mash the sweet potatoes with a fork or potato masher until smooth.
Add the black beans to the bowl and mash them slightly, leaving some whole for texture.
Stir in the breadcrumbs, corn, red onion, garlic, cumin, chili powder, salt, pepper, lime juice, and cilantro (if using). Mix until well combined.
Form the mixture into patties, about 3 inches in diameter. You should get around 8 patties.
Heat olive oil in a large skillet over medium heat. Once hot, add the patties in batches, cooking for about 4-5 minutes on each side until golden brown and crispy.
Remove from the skillet and place on a paper towel-lined plate to absorb excess oil.