Start cooking your rice. Cut chicken into 1" pieces.
Mix together the flour, cornstarch, salt, pepper, and cayenne pepper in a shallow dish. Dredge chicken in flour mixture.
Cook chicken in the hot oil in a large skillet over medium-high heat till the outside is golden brown and the internal temperature is 165 degrees. Remove to a plate.
While the chicken is cooking, whisk the water, sugar, cornstarch, soy sauce, hoisin, vinegar, garlic, and ginger in a small bowl. Add to the skillet; cook and stir over medium heat till thick and bubbly.
Turn the heat to low and return the cooked chicken to the pan. Spoon the sauce over the top of each piece and simmer for 2-3 minutes.
Serve chicken and sauce over hot fluffy white rice. Garnish with green onions and sesame seeds.
Notes
This dish is best served immediately, because as it sits the chicken coating softens. You can store leftovers in an airtight container in the fridge for 2-3 days, but the breading tends to fall apart.