Creating delectable Shoyu Tamago is straightforward if you follow these steps:

1. Boil the Eggs: Bring a pot of water to a gentle boil. Carefully add the eggs and boil for exactly 6-7 minutes for a soft-boiled texture.
2. Prepare Ice Bath: While the eggs are boiling, prepare an ice bath in a bowl filled with ice and cold water to shock the eggs after cooking.
3. Shock the Eggs: Once the timer goes off, remove the eggs from the boiling water and immediately submerge them in the ice bath for about 5 minutes to stop the cooking process.
4. Mix Marinade: In a separate bowl, combine soy sauce, mirin, water, and sugar (optional). Stir until the sugar dissolves.
5. Peel the Eggs: Once cooled, gently tap the eggs on a hard surface and peel the shells carefully.
6. Marinate the Eggs: Place the peeled eggs in a container and pour the marinade over them, ensuring they're fully submerged.
7. Refrigerate: Seal the container and refrigerate the eggs for at least 2 hours. For best results, marinate overnight.
8. Serve: After marinating, remove the eggs from the marinade and slice them in half to reveal the beautiful yolks.

These steps will guide you through making Shoyu Tamago quickly and efficiently!