Cut the chicken into evenly sized bite-size pieces and toss with cornstarch, salt, and black pepper until evenly coated.
Heat the vegetable oil in a large pan over medium-high heat. Cook the chicken in a single layer until golden and cooked through, then remove and set aside.
Reduce the heat to medium and add the garlic and ginger to the same pan. Stir briefly until fragrant.
Add the honey, soy sauce, vinegar, and water. Stir well and let the sauce simmer until slightly thickened.
Return the chicken to the pan and toss until every piece is coated in the sauce.
Stir in the sesame oil and sprinkle with sesame seeds. Remove from heat and garnish with green onions if using.