Prepare the sushi rice in a rice cooker according to the manufacturer's instructions, adding about 15% more water than usual for perfect stickiness. Once cooked, transfer the rice to a bowl, and sprinkle in fine salt and furikake. Gently fold together until combined.

Dice chicken into small, bite-sized pieces. In a bowl, mix soy sauce, sake, baking soda, and cornstarch. Add chicken to the marinade, coating well, and let it sit for about 30 minutes.

In a small bowl, whisk together soy sauce, mirin, and brown sugar. Set this mixture aside.

Heat oil in a pan over medium-high heat. Add the marinated chicken and sauté for about 2-3 minutes until browned and cooked through.

Reduce heat and add minced garlic, sautéing for 30 seconds. Pour in teriyaki sauce over chicken and cook for 2-3 minutes until the sauce thickens.

Wet your hands to prevent sticking. Take about ⅓ cup of rice and press into your palm, creating a well in the center. Spoon in 1 tablespoon of the chicken mixture, then cover it with another ⅓ cup of rice. Mold the rice into a triangular shape.

Wrap each onigiri with a piece of nori. Let the shaped onigiri rest for a few minutes before serving.