Fit food processor with shredding disc. Shred sweet potatoes and place in colander. Shred onion and toss with sweet potatoes. Let stand to drain for 10 minutes. Transfer to a large bowl.
Whisk together chili powder, eggs, salt and pepper. Stir into sweet potato-onion mixture. Sift potato starch into the mixture and mix well.
In a large nonstick skillet, heat oil over medium-high heat. Place 6 mounds of mixture (about N cup each) into the skillet. Press down lightly. Sauté until browned, about 2 to 3 minutes per side (cook in 3 batches). Drain on a baking sheet lined with paper towels. Sprinkle with salt to taste. Serve with sour cream, if desired.