In a small saucepan, whisk together soy sauce, 1/4 cup plus 2 tablespoons water, brown sugar, honey, garlic, ginger, and rice vinegar.
Bring the mixture to a gentle boil over medium-high heat.
In a small bowl, whisk cornstarch with the remaining 2 tablespoons water.
Add the cornstarch slurry to the saucepan and stir continuously.
Simmer for 1 minute until the sauce thickens.
Remove from heat and set aside.
Grill the Chicken
Brush both sides of the chicken breasts with 1 1/2 tablespoons olive oil.
Season lightly with black pepper.
Preheat a grill to medium-high heat, about 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).
Remove from the grill and allow to rest for 5 minutes.
Dice into bite-sized cubes.