In a large Ziploc bag, add the chicken chunks, flour, cornstarch, salt, pepper, and garlic powder. Shake until the chicken is well coated.
In a large skillet over medium-high heat, heat the vegetable oil (to about 350 degrees). Test to see if the oil is ready by adding one of the chicken chunks. If it's ready, the oil will sizzle around the chicken.
Add the rest of the chicken chunks (shake off excess flour mixture before adding to the skillet) evenly in the skillet (don't overcrowd the pan or the temperature of the oil will drop, resulting in soggy chicken). You can cook in batches if you need to.
Let the chicken cook until well browned on each side, about 5 minutes total.
Remove the chicken from the oil with a slotted spoon and transfer to a plate lined with paper towels to drain.
Make the sauce while the chicken is cooking: In a small saucepan over medium heat, add the vegetable oil.
Add the garlic, ginger, and red pepper flakes and cook until fragrant, about one minute.
Add in the soy sauce, water, hoisin sauce, brown sugar, and cornstarch. Stir until well-combined. As the mixture starts to simmer, it will thicken. Lower the heat to low and allow it to gently simmer for 5 minutes.
Add the cooked chicken to a bowl and pour the hot sauce over the chicken chunks. Gently toss to combine and serve hot over cooked rice and sauteed vegetables.