In a small bowl, toss the cubed chicken with the cornstarch. Heat oil in a skillet over medium heat. Add chicken and cook until browned. Add the broccoli and continue cooking until the broccoli softens. Lower heat to a simmer.

Whisk all remaining ingredients except for sesame seeds together in a bowl. Pour the mixture into the skillet and toss with to combine with the chicken and broccoli. Simmer until the sauce thickens, about 10 minutes.

Heat a small pan over low heat. Add the sesame seeds and cook until they start to turn a golden color and become fragrant. Make sure to move the seeds around the pan to toast them evenly. Remove from heat.

Distribute rice among 4-5 serving bowls. Top with the chicken and broccoli, toasted sesame seeds and diced scallions.