Wash & slice sweet potatoes in 1/4 inch wide sticks. Soak sliced potatoes in a bowl of cold water for an hour. Pour out the water, rinse & pat dry. Line 2 baking sheets with foil, parchment, or something that won't stick to the potatoes. Spray with non-stick spray.
Melt a stick of butter in large bowl or pan. Add salt, chili powder and pepper to melted butter & set aside.
Place 1/2 of your (patted dry) potato sticks into a giant zipper top bag and sprinkle in 1 tbsp starch. Shake it all up and dump your starched potato sticks into the bowl of seasoned butter. Toss them around in the butter mixture to coat & arrange them in a single layer on one of the baking sheets.
Next bag & starch the other half of the potato sticks, then toss them in the butter mixture & arrange on the second baking sheet.
Bake in 425 degree oven for 15 mintues. Put some space between the 2 sheets - you want everyone to have room to crisp up.
After 15 mintues, flip all the taters over & bake another 10 mintues (total baking time: 25 mintues).
Once they are done, remove from the oven and let cool slightly. Add more salt to taste. Serve warm, not hot. This will make them crispier.
I think the best sauce for sweet potato fries is a simple Russian Dressing. Your favorite mayo with a little ketchup stirred in. Yum!