In a small sauce pot, mix soy sauce, mirin or sugar/vinegar, and dissolved dashi stock or water. Bring mixture to a boil, stirring occasionally for 5 minutes or until the sugar has dissolved (if you’re not using mirin).
Remove marinade from the heat and set aside to cool completely, about 10 minutes.
Fill a medium pot with enough water to cover three eggs. This will depend on the size of your pot–I filled mine ¾ of the way and that was enough.
Bring water to a rolling boil.
In a separate bowl, add 1 cup ice + 2 cups water. This is the cold bath that will stop your eggs from overcooking inside the shell and make sure they remain soft-boiled.
Gently place eggs in the water, careful not to crack them.*
Boil the eggs for exactly 6 minutes.
Remove eggs from boiling water and place in ice bath to cool for about 5 minutes.
When your eggs have cooled, gently peel the shell off the egg, making sure not to tear the egg itself. Place “naked” eggs in a ziploc bag.
Pour the completely cooled marinade over the eggs in the Ziploc bag.
Seal the bag and store upright in a bowl in the fridge so that the eggs are completely covered. When you’re ready to serve, remove desired number of eggs and let sit at room temperature for 5-7 minutes before slicing.