Rinse sushi rice under cold water until clear, combine with water and salt, then cook according to rice cooker's instructions. Let sit covered for 10 minutes.

Cut chicken into bite-sized pieces, marinate with soy sauce, sake, baking soda, and cornstarch for 30 minutes in the refrigerator.

Whisk together soy sauce, mirin, and brown sugar in a bowl until dissolved to make the teriyaki sauce.

Heat oil in a nonstick pan, sauté marinated chicken for 2-3 minutes until golden, add garlic, then pour in teriyaki sauce and cook until thickened.

Wet hands, scoop rice into palm to form a ball, fill with teriyaki chicken, cover with more rice, and shape into a triangle.

Wrap nori around each onigiri and serve immediately or pack them for lunch.