First, beat the egg and combine it with the water in a small shallow bowl, and stir.
Next, combine the flour, tempura mix, salt, pepper, onion powder, and garlic powder in a one-gallon size ziplock bag.
Pound each of the chicken breast filets with a mallet until they're about 1/4-inch thick. Trim each breast filet until it is round. Coat each filet with the flour mixture by shaking it in the zip lock bag.
One at a time, remove a chicken breast and dredge each filet in the egg mixture, making sure to coat them well. Then return each filet to the flour/seasoning mixture. Shake to coat.
Place the chicken filets, bag and all, in the freezer for at least an hour. Also, cover and refrigerate the remaining egg mixture. After freezing, repeat the flouring process.
Heat a frying pan to 375°. Deep fry the chicken filets for 10-12 minutes or until light brown and crispy.
While the chicken is frying, toast the hamburger buns and place the mayo sauce inside of the top bun. Then place 1/4 of a cup of freshly chopped iceberg lettuce on the bun, followed by a slice of tomato. Add one of the cooked chicken patties, and close it with the other half of the bun. Serve hot, and enjoy.