Prepare the chicken. Add the chicken thighs, salt, black pepper, cornstarch, baking powder, and egg to a large mixing bowl. Mix thoroughly until every piece is evenly coated. Set aside for a few minutes.
Heat the oil. Pour enough vegetable oil into a wok or deep, heavy-bottomed pan for the chicken pieces to float. Heat over medium-high heat until the oil reaches about 175°C / 350°F.
First fry. Carefully add the chicken pieces one at a time, working in batches so the pan does not become overcrowded. Fry for 3–5 minutes, or until golden and cooked through. Transfer to a wire rack or paper towel-lined plate and repeat with the remaining chicken.
Second fry. Return the fried chicken to the hot oil and fry for about 1 minute, until deeply golden and extra crispy. Drain well.
Make the sauce. Heat a clean wok over medium heat and add the butter. Once melted, add the minced garlic and stir for about 15 seconds, until fragrant.
Add the soy sauce, brown sugar, and honey. Stir for about 20 seconds, then reduce the heat to low.
Coat the chicken. Add the crispy chicken to the sauce and toss gently until every piece is evenly coated. Sprinkle in the toasted sesame seeds and mix again.
Serve. Transfer the chicken to a serving plate. Sprinkle with additional sesame seeds and chopped green onions. Serve immediately while hot and crispy.