Rinse sushi rice under cold water until water runs clear. Combine with water in a pot, bring to a boil, reduce heat, cover, and simmer for 15 minutes. Let sit covered for 10 minutes.

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

In a saucepan, whisk together soy sauce, mirin, and brown sugar. Simmer until sauce thickens, about 5 minutes. Remove from heat.

Heat oil in a frying pan, add marinated chicken in a single layer. Stir-fry for 5-7 minutes until golden brown and cooked through.

Add minced garlic to the skillet and stir for 1 minute. Pour teriyaki sauce over chicken, stir well, and cook for another 2 minutes.

With wet hands, take ⅓ cup of rice, form into a ball, create a well, and fill with teriyaki chicken. Top with another ⅓ cup of rice and shape into a triangle.

Wrap a piece of nori around the base of the rice triangle, pressing gently to secure.

Serve on a platter, optionally with teriyaki sauce and furikake on the side.