Prepare the chicken marinade by whisking together ginger, garlic, soy sauce, black pepper, and salt in a large bowl. Add chicken breasts, ensuring they're well-coated, then cover with plastic wrap and refrigerate for at least 30 minutes, preferably overnight for maximum flavor.
Create the cornstarch crumble by adding cornstarch to a shallow bowl. Gradually, mix in 1 tbsp. of water at a time until crumbles form, repeating this step around 8 times until half the cornstarch is transformed.
Once the chicken is marinated, add flour and egg, mixing thoroughly. Lightly press the chicken into the cornstarch crumble, shaking off the excess, then transfer to a wire rack.
Heat 4 cups of oil to 350°F. For the first-fry, gently submerge 4 to 5 chicken pieces at a time, ensuring not to overcrowd the pot. Fry for 2-3 minutes until lightly golden, adjusting heat if necessary. Transfer the fried chicken to a wire rack and repeat with the remaining pieces, cleaning oil between batches.
For the second fry, raise the oil temperature to 380°F. Deep-fry 4 to 5 pieces of chicken for another 1-2 minutes until golden brown and crispy. Drain the excess oil on a wire rack and repeat with the remaining chicken.