In a medium bowl, whisk together the cornstarch and flour with 1 teaspoon kosher salt and 1/8 teaspoon pepper. Add the chicken pieces to the bowl, tossing to coat. 

Add 2 to 3 inches of vegetable oil to a large heavy-bottomed stockpot set over medium-high heat. Line a plate with paper towels. 

Once the oil is hot, shake off any excess flour mixture from the chicken pieces and add them to the oil in batches. (See Note.) 

Cook the chicken pieces in batches, turning as needed, until golden brown and cooked through. Using a slotted spoon, transfer the cooked chicken to the paper towel-lined plate then make the sauce.

In a small bowl, whisk together the cornstarch and soy sauce to make a slurry. Set the slurry aside.

Add the vegetable oil to a large saucepan set over medium heat. Once the oil is hot, add the chilies, garlic and ginger and cook, stirring constantly, until the garlic is golden brown, about 1 minute.

Stir in the rice wine vinegar, hoisin, brown sugar, chicken stock and sesame oil. 

Stir in the slurry, bring the sauce to a simmer and cook it until it thickens to the consistency of syrup, about 1 minute.

Return the chicken to the pan and toss to coat. 

Serve the chicken topped with scallions.

You can test if the oil is hot enough by adding one piece of the chicken to the pan. If it bubbles and floats, the oil is the correct temperature.

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