In a small bowl, whisk together the listed sauce ingredients until the sugar fully dissolves. Set aside.

Dice the chicken thighs into even 1.5-inch pieces. Don't pat them dry; the natural surface moisture helps the cornstarch stick.

Place the chicken in a large bowl. Season with salt and black pepper. Add the cornstarch, and toss until evenly coated. If wet spots remain, add 1-2 tablespoons more cornstarch as needed.

Heat the oil in a large, heavy-bottomed pan over medium-high heat to 350°F. Fry the chicken in small batches for 5-7 minutes total. Don't move the chicken until the bottom is golden and the crust has set; then flip and fry until the other side is golden and the chicken reaches 165°F. Between batches, return the oil to 350°F and remove browned bits. Drain on a wire rack to keep the crust crisp.

Remove any remaining oil from the pan and reduce the heat to medium. Whisk the sauce again, pour it into the pan, and simmer for 30-45 seconds or until thick enough to coat the back of a spoon.

Quickly toss the fried chicken in the thickened sauce. Garnish with sesame seeds and lemon slices, then serve immediately.