In a small bowl, whisk together all the cornstarch slurry ingredients until the cornstarch is fully dissolved. Set aside.
Dice the chicken thighs into uniform 1.5-inch pieces. Do not pat the chicken dry, as the moisture helps the cornstarch stick.
Transfer the chicken to a large mixing bowl and add the cornstarch for coating. Toss until evenly coated. If any areas remain uncoated, add 1-2 tablespoons more cornstarch as needed.
Heat vegetable oil in a large, heavy-bottomed pan over medium-high heat to 350°F. Fry the chicken in a single layer, working in batches if needed, for about 5-7 minutes per batch. Leave the chicken undisturbed until the bottom is golden and the crust has set before flipping. Continue frying until the internal temperature reaches 165°F.
Transfer the fried chicken to a wire rack to keep it crispy.
Remove the excess frying oil, leaving 2 teaspoon (10 ml) in the pan. Reduce heat to medium. Stir-fry the ginger, garlic, and half of the red chilies for 10 seconds.
Add the hoisin sauce, soy sauce, brown sugar, and toasted sesame oil. Stir to combine. Stir the cornstarch slurry again, then pour it in. Simmer until thickened and glossy.
Add the chicken and toss to coat evenly. Remove from heat, garnish with the remaining red chilies and green onions, and serve immediately.