Despite their similarities as thickening and coating agents, cornstarch and flour are not interchangeable on a one-to-one basis in all cooking. The two products -- and tapioca, another thickener you might have in your cupboard -- behave differently in dishes, depending on the cooking method you use. To make the best sauces, stir-fries, fruit pies, fried fish fillets and schnitzel, use flour for cornstarch in specific proportions for cooking specific foods.
Because cornstarch has far fewer protein molecules than flour -- the gluten, a protein, in flour help bread keep its structure -- cornstarch produces a lighter, crispier crust on fried chicken or fish, sweet potatoes or tofu. That said, you can substitute flour for cornstarch on a one-to-one basis when coating those foods, knowing that the coasting will be less crunchy. Other thickeners, such as tapioca, don't work at all for coating foods.
Add 1/2 teaspoon of baking powder to each cup of either flour or cornstarch when you coat foods for frying. Baking powder creates air bubbles in the coating that increase the surface area of the food and result in a crunchier coating.