Pat the chicken pieces very dry with paper towels. In a large bowl, toss chicken with salt and black pepper until evenly coated. In a separate shallow dish, whisk together cornstarch, flour, and baking powder. Dredge each piece in the flour mixture, shaking off excess, and place on a wire rack. Let sit while oil heats.

In a small saucepan over medium heat, combine gochujang, soy sauce, honey, brown sugar, sesame oil, garlic, and ginger. Stir in water and bring to a gentle simmer, then reduce heat to low and cook for 3-4 minutes until thickened slightly. Remove from heat and set aside.

Heat oil in a large heavy-bottomed pot or deep fryer to 330°F (165°C). Working in batches so as not to crowd the pot, fry chicken pieces for 6-7 minutes until just cooked through (internal temperature reaches 165°F/74°C) and lightly golden. Transfer to a wire rack set over a baking sheet and let rest for at least 10 minutes.

Increase oil temperature to 375°F (190°C). Fry the rested chicken again in batches for 5-7 minutes until deeply golden and the skin is shatteringly crisp. Drain on fresh paper towels or a clean wire rack.

While chicken is still hot, place it in a large clean bowl. Pour the warm sauce over the top and gently toss with a spatula until each piece is evenly coated. Transfer to a serving platter and scatter generously with sesame seeds and sliced green onions.