Cut the chicken into 1-inch pieces. Add the chicken and ½ cup cornstarch to a large bowl. Toss the chicken with the cornstarch until fully coated. Remove the chicken from the cornstarch, shaking off the excess.
Whisk together the soy sauce, hoisin sauce, rice vinegar, brown sugar, water and remaining 1 1/2 tablespoons cornstarch until the sauce is smooth and no lumps remain. Set aside.
Add the oil to a 9 inch or larger skillet over high heat. Make sure you add enough oil to fully coat the bottom of the pan, you may need a little more or a little less than called for in the recipe, depending on the size of your pan.
When the oil is hot, add the chicken. Make sure the chicken is in a single layer, with all pieces touching the pan. You may need to cook the chicken in two batches. Fry the chicken for 4-5 minutes per side, until fully cooked through. Remove the chicken from the pan and set aside.
Wipe out the skillet and turn the heat down to medium. Add an additional teaspoon of oil to the skillet with the minced garlic and ginger. Cook 30 seconds.
Pour the sauce mixture into the skillet with the ginger and garlic, and add a pinch of red pepper flakes. Stir the sauce for 4-5 minutes, until it thickens. The sauce is thick enough when you can trace a trail in it with a spoon.
Return the chicken to the pan and coat it in the sauce. Top with green onions and sesame seeds.