Heat broiler. In small skillet over med heat, warm 1 T oil. Cook scallions til soft, 1 minute Add garlic, jalapeno and cumin. Cook til fragrant, 30 seconds. Transfer to large bowl.
Add beans to bowl; mash with fork or potato masher, leaving about 1/4 of the beans whole. Season generously with salt and pepper. Fold in sweet potato, egg and breadcrumbs. Divide into 8 balls of equal size; flatten into patties.
Brush a baking sheet with remaining T oil; place patties on sheet, 1/2 inch apart. Broil 4 inches from heat til golden brown, 8-10 minute With thin metal spatula, carefully turn cakes. Broil til crisp, 2-3 min more.
If desired serve with Lime Sour Cream.
Lime Sour Cream: In small bowl, combine 1/2 c sour cream with 2 t fresh lime juice and 1 small minced jalapeno; season with coarse salt.