Whisk buttermilk, egg and 1/2 cup water in a medium bowl. Whisk flour, cornstarch, remaining 1 Tbsp. salt and remaining 1 Tbsp. pepper in a shallow bowl.
Pour oil into a 10-12" cast iron skillet to a depth of 1/2". Heat oven medium-high heat until temperature reaches 350 degrees F.
Dip chicken in buttermilk mixture, allowing excess to drip back into bowl. Dredge in flour mixture; shake off excess. Place 4 or 5 pieces of chicken in skillet. Fry, turning once until golden brown and chicken is cooked through- approx. 2-3 minutes per side. Remove chicken and transfer to a cooling rack.