Place a steamer basket in a medium saucepan filled with 1 inch of water. Bring to a boil, and add sweet-potato cubes. Steam until fork tender, 6 to 7 minutes. Set aside. Place a large skillet over medium heat. Spray skillet with cooking spray. Add onion, garlic, curry powder, and cumin; cover. Cook, lifting cover to stir occasionally, until translucent, about 5 minutes. If the onion begins to dry out or burn, add 2 tablespoons water. Stir in tomato paste, and cook another 2 minutes.
Transfer mixture to the bowl of a food processor. Add the sweet-potato cubes and the potato starch. Pulse until onions are finely ground, about 1 minute.
Rinse and drain the tofu. Lay a clean kitchen towel or a double thickness of cheesecloth on a clean work surface. Place tofu in the center, wrap, and wring out any excess liquid; the tofu should have the texture of ground meat. Place the tofu, sweet-potato mixture, brown rice, chives, cilantro, salt, and pepper in a medium bowl. Using a rubber spatula, fold the ingredients together. Using your hands, shape the mixture into four 3 1/2-inch-round patties.
Return skillet to medium heat. Coat with cooking spray. Place patties in pan, and cook until golden on bottom, 5 to 6 minutes. Flip patties over, and cook until golden on other side, another 5 to 6 minutes. Serve warm on whole-wheat buns.