Cut the chicken into bite sized pieces. Toss the chicken in the mirin to coat.
Combine potato starch, garlic powder and salt in a small bowl. Toss the dry mixture into the chicken so that the chicken is coated. Cover and refrigerate for at least 1 hour (up to overnight).
Heat 3/4" of vegetable oil in a heavy bottomed pan. If you have a food thermometer, aim for an oil temperature of 175 degrees. If not, set the burner to medium heat.
Drop the chicken pieces into the oil and fry until slightly browned, about 8-10 minutes. Remove with a slotted spoon onto a paper towel-lined plate. Wait 1-2 minutes, then put the chicken back into the oil for an additional 2-3 minutes, until golden brown. Remove onto paper towels again.
Combine soy sauce, honey, brown sugar, ginger, garlic and red pepper flakes in a small saucepan. In a separate bowl, make a slurry of 1 Tbsp. cornstarch and 2 Tbsp. water.
Whisk the sauce ingredients together over medium heat until foamy. Watch carefully so that it doesn't boil over.Pour in the cornstarch slurry and whisk. Lower the heat. Simmer for 5-7 minutes, until sauce is thickened.
Toss the chicken in the soy garlic sauce in a large bowl. Garnish with scallions and sesame seeds.