Rinse rice thoroughly until it is clear. Cook it in a rice cooker or pot.
After cooking, mix rice with vinegar, 2 tbsp sugar, and salt. Let cool.
Clean and shape chicken to ½-inch thickness. Make 8 pieces, each about 2 inches wide.
In a large wok, heat sake, soy sauce, and remaining sugar. Bring to a simmer and cook until slightly thickened.
Combine the chicken and soy sauce mixture. Until fully cooked, cook each side for 5 minutes.
Let the chicken cool on a plate.
Now, assemble musubi. In front of you, place the nori strip vertically.
Put musubi mold on the top-middle of nori.
Add ¼ cup rice to the mold and press firmly with a wet top.
As needed, sprinkle furikake on top. Add chicken and furikake to taste.
Scoop ¼ cup rice into the mold and press again.
Use both hands to press the musubi from the top to the bottom. Remove the mold's center part.
Fold over Nori on both sides.
Cut in half or enjoy it whole. Now, savor your Chicken Musubi!