In a medium bowl, combine the chicken pieces and buttermilk. Mix well and let marinate for 15 minutes.
In a separate large bowl, whisk together cornstarch, whole wheat flour, baking powder, salt, black pepper, garlic powder, and onion powder.
In a small bowl, beat the egg.
Drain excess buttermilk from the chicken pieces, then dip each piece into the beaten egg, followed by the flour mixture. Ensure each piece is evenly coated.
Heat olive oil in a large non-stick skillet over medium-high heat.
Add the coated chicken pieces in a single layer, cooking for 3-4 minutes on each side until golden brown and cooked through. Work in batches if necessary to avoid overcrowding.
Remove chicken from the skillet and place on a paper towel-lined plate to drain excess oil.
In a small saucepan, combine gochujang, honey, soy sauce, rice vinegar, minced garlic, grated ginger, sesame oil, and water. Bring to a simmer over medium heat, stirring frequently for 2-3 minutes until sauce is well blended and slightly thickened.
Place the cooked chicken in a large bowl, pour the sauce over, and toss to coat evenly.
Transfer the coated chicken to a serving plate, sprinkle with toasted sesame seeds and sliced green onions.
Serve immediately.