First, combine chicken breast and teriyaki marinade in a bowl and stir to coat. Marinate at room temperature for 15 to 20 minutes. In the meantime, lightly grease and preheat a cast iron grill pan over medium-high heat.
Next, bring the water to a boil in a small saucepan. Stir in white rice, then cover and turn heat to low. Steam until the rice has absorbed all the water, about 20 minutes.
Transfer the chicken to the hot grill pan. Grill until cooked through completely, flipping over every 3 minutes, about 10 to 12 minutes total. Then transfer to a cutting board and let rest for 2 to 3 minutes.
In the meantime, toast sesame seeds in a small frying pan over medium heat until golden, about 5 to 7 minutes. Shake the pan every few minutes as they toast to make sure they heat evenly.
Finally, slice the chicken into thin strips and plate on top of white rice. Drizzle with Takumi triple ginger teriyaki sauce and top with toasted sesame seeds and scallions. Serve immediately.