In a small saucepan, whisk together soy sauce, ¼ cup plus 2 tablespoons water, brown sugar, honey, garlic, ginger, and rice vinegar.
Bring to a gentle boil over medium-high heat.
In a small bowl, whisk cornstarch with the remaining 2 tablespoons water until smooth.
Stir the cornstarch mixture into the sauce.
Boil for 1 minute while stirring constantly until thickened.
Remove from heat and set aside.
Grill the Chicken
Brush both sides of the chicken breasts with 1½ tablespoons olive oil.
Season lightly with black pepper.
Preheat the grill to 425–450°F (220–230°C).
Lightly oil the grill grates.
Grill chicken for approximately 4 minutes per side or until the internal temperature reaches 165°F (74°C).
Let rest for 5 minutes, then cut into bite-sized pieces.