Make the spice mix by combining all the ingredients together. Start with 1/8 teaspoon of salt first then add later to your liking (Note 1). Set aside.
Slice sweet potatoes into thin, uniform fries. In a mixing bowl, whisk cornstarch and cold water. Dip the fries in the batter. Do in batches, if necessary. Allow any excess to drip off by placing the dipped pieces in a wire rack.
Heat a heavy-bottomed pan with oil over medium heat until about 350 degrees F. The amount of oil should be enough for the the sweet potatoes to be completely submerged. Fry the sweet potatoes, in batches if necessary, until golden brown and crispy (around 3-5 minutes). Immediately sprinkle with the spice mix.
Notes
(Note 1) Try the spice mix first by adding it into few pieces of cooked sweet potato fries. Adjust the taste (e.g. add salt, cumin) according to your liking before using the whole batch. The amount of cayenne in this recipe just adds an extra kick. If you want it to be spicy, add more or use chili powder.