Marinate the chicken: In a bowl, combine 1.5 pounds boneless chicken thighs, cut into 1-inch pieces with 2 tablespoons soy sauce (for marinade), 1 large egg, 0.5 teaspoons salt, 0.3 teaspoons black pepper, and 1 teaspoons garlic powder. Mix well and let sit while the air fryer heats.
Coat in starch: Add 0.5 cups potato starch and toss until every piece is coated in a thick, slightly tacky batter. Add a little more starch if it looks wet.
Air fry: Preheat the air fryer to 400°F (200°C). Arrange chicken in a single layer without crowding (work in batches if needed) and spray generously with oil.
Flip and finish: Flip the pieces, spray any dry or powdery spots with more oil, and cook until golden, crisp, and cooked through (165°F internal).
Make the sauce: While the chicken cooks, melt 1 tablespoons butter in a pan over medium heat. Add 5 garlic cloves, minced and cook until fragrant, about 3 minutes. Stir in 0.3 cups gochujang, 2 tablespoons soy sauce (for sauce), 2 tablespoons brown sugar, 2 tablespoons honey, 1 tablespoons rice vinegar, and 1 tablespoons ketchup. Simmer until glossy and slightly thickened.
Toss and serve: Remove the sauce from the heat and stir in 1 tablespoons sesame oil. Add the hot chicken and toss to coat. Serve right away while crispy.