In a medium bowl, whisk together the soy sauce, water, brown sugar, mirin, rice vinegar, ginger, garlic, and sesame oil for the sauce. Set aside.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the cut chicken pieces and cook until browned on all sides and cooked through, about 6 to 8 minutes. Remove the chicken from the skillet and set aside.
Pour the prepared sauce mixture into the same skillet. Bring the sauce to a simmer over medium heat, stirring constantly. Let it simmer for 2 minutes.
Whisk the cornstarch slurry one more time and slowly pour it into the simmering sauce while stirring continuously. The sauce will thicken quickly. Continue to cook for 1 minute until the sauce is glossy and thick enough to coat the back of a spoon.
Return the cooked chicken to the skillet. Toss the chicken pieces gently until they are completely coated in the thick, sticky teriyaki glaze.
To assemble the bowls, divide the cooked rice among four bowls. Top the rice with the sticky teriyaki chicken and arrange the cooked broccoli alongside.
Garnish the chicken with sesame seeds before serving.
Notes
For extra sticky sauce, let the sauce simmer for an extra minute after adding the slurry until it reduces slightly more.
Chicken thighs hold moisture better than breasts, resulting in juicier chicken for bowls.
If you prefer a sweeter glaze, add one extra tablespoon of brown sugar to the sauce mixture.