In a large bowl, combine the flour, cornstarch, baking powder, salt, black pepper, garlic powder, and ginger powder. Mix well and set aside.
In another bowl, pour the buttermilk over the chicken pieces, ensuring they are well-coated. Let them marinate for at least 30 minutes in the refrigerator.
Heat the vegetable oil in a deep skillet or frying pan over medium-high heat until it reaches 350°F.
Remove the chicken from the buttermilk, allowing excess liquid to drip off. Dredge the chicken pieces in the flour mixture, ensuring they are fully coated.
Carefully place the coated chicken pieces into the hot oil, frying in batches to avoid overcrowding. Fry for about 6-8 minutes or until golden brown and cooked through, reaching an internal temperature of 165°F.
Use a slotted spoon to transfer the fried chicken to a paper towel-lined plate to drain excess oil.
Serve hot with lemon wedges and garnish with chopped green onions.