Slice the chicken against the grain into thin strips, then add to a bowl with 2 tablespoons cornstarch, 1 tablespoon light soy sauce, and 1 tablespoon oil. Mix well. Marinate for at least 30 minutes. If marinating for longer, do so in the refrigerator.
Make the stir-fry sauce by combining all the sauce ingredients in a medium saucepan and placing it over medium heat. Heat the sauce while stirring until the sugar dissolves.
In a small bowl, stir 1 tablespoon of cornstarch with 3 tablespoons of cold water. Set aside.
Heat a large, heavy skillet or wok over high heat. Add 1 tablespoon of high-heat oil (like vegetable or avocado oil).
Add the marinated chicken in a single layer. Let it cook for one minute without moving until the underside starts to brown.
Toss the chicken and continue cooking until it’s almost cooked, 3 to 5 more minutes. Remove the chicken from the pan and set aside.
Add another tablespoon of oil to the hot pan. Stir in the broccoli florets and cook, stirring frequently, until bright green and crisp-tender, 3 to 5 minutes.
Return the chicken to the pan and toss with the broccoli for about a minute.
Pour in the stir-fry sauce. Give the cornstarch mixture another quick stir, then slowly add it to the pan.
Stir constantly until the sauce thickens and coats the chicken and broccoli. Serve immediately.
