Slice sweet potatoes very thin. Use a mandolin if possible. They must be the same thickness (or thinness in this case). The thinner they are the crispier they will be.
Soak the sweet potato chips in cold water for at least 30 minutes. Then use a colander to drain them and dry them well with paper towels. This allows the starch to come out and the sweet potato chips will bake more evenly and get crispier.
In a medium mixing bowl, mix the oil, cornstarch, garlic, onion powder, and pepper. Add the sweet potatoes and mix everything well so that the sweet potatoes are coated.
Prepare the air fryer by brushing it with cooking oil. Place the sweet potato chips into the basket as evenly as possible. It is okay if they overlap a bit because they will shrink.
Bake them at 400 for 5 minutes. Then remove the basket and give the sweet potato chips a shake. Then bake the chips for another 4 minutes at 350 degrees.
Remove them from the basket and put them on a cooling rack. This will help them crisp up. While they are still warm, season them generously with seasoning salt.