Prepare marinade by mixing buttermilk, sugar, and salt in a large bowl. Add the chicken and coat well. Cover and refrigerate about 4 hours or overnight. Remove chicken from marinade and pat dry. Allow chicken to sit at room temperature about 30 minutes on a cooling rack over a cookie sheet before frying.
Prepare batter by whisking together flour, cornstarch, baking soda, salt, and pepper in a large bowl. Slowly whisk in water.
Heat the oil to 350°F in a large pot or dutch oven. Place half the chicken in the batter and remove one piece at a time to the oil. Fry the chicken to a deal golden brown, about 12 minutes. Adjust burner level as needed to keep oil temperature even.
Place chicken on paper towels to drain. Transfer to a wire rack set over ea cookie sheet and sprinkle with a small amount of salt. Repeat with remaining chicken.