Pat the chicken pieces dry with a paper towel. In a mixing bowl, whisk together the flour, cornstarch, salt, black pepper, and garlic powder until evenly combined.
Add the chicken pieces to the bowl and toss until every piece is lightly and fully coated. Shake off the excess coating from each piece.
Heat the avocado oil in your skillet over medium-high heat until it shimmers. Add the chicken in a single layer, working in two batches if the pan looks crowded.
Fry undisturbed for 3 to 4 minutes, then flip with tongs and fry the other side for another 3 to 4 minutes. Cook until golden brown and reach an internal temperature of 165°F (74°C). Transfer to a wire rack.
Lower the heat to medium and wipe out any burnt bits, leaving the pan lightly greased. Add the butter and let it melt, swirling the pan as it starts to foam and turn a shade golden.
Stir in the minced garlic and cook for about 30 seconds, just until fragrant. Pour in the honey, soy sauce, rice vinegar, and red pepper flakes if using. Simmer for 1 to 2 minutes, stirring, until the sauce looks glossy and slightly thickened.
Return the fried chicken to the skillet. Toss gently with tongs or a spatula until every piece is coated in the sticky glaze. Let it sit in the sauce for about 30 seconds, then remove from the heat.
Scatter toasted sesame seeds and sliced green onions over the top. Serve right away while the crust is at its crispest, spooning any extra sauce from the pan over the chicken.