Combine ¼ cup soy sauce, ¼ cup mirin, ¼ cup sake, and 1 tsp sugar in a small saucepan. Whisk to dissolve the sugar completely. Bring it to a boil, then lower the heat. Simmer for 1 minute. Turn off the heat. Set aside to cool completely.

Add 4 cups (1L) water to a saucepan. Make sure there's enough water to cover the eggs by 1 inch (2.5 cm) once added. Bring the water to a full boil. Remove 4 large eggs (50 g each w/o shell) from the fridge. Gently lower one egg at a time into the boiling water with a ladle or strainer. When you add the first egg, set a timer for 7 minutes.

After 7 minutes, promptly remove the eggs. Submerge them in a bowl of iced water for 15 minutes until cooled completely.

Gently crack the eggshell at the wide bottom end. Start peeling it vertically toward the pointy top. Dip the egg in the iced water a few times to help with peeling.

Place the peeled eggs in a plastic bag. Add the cooled marinade. Remove the air from the bag. Seal the bag right above the eggs with a clip or rubber band to keep them completely submerged. Refrigerate for 8 hours or overnight.

Remove the eggs from the marinade. Cleanly cut them in half lengthwise with a fishing line or wire cheese slicer. If you want to warm them, first soak the bag in warm water to bring up the temperature.