Fill a medium saucepot with enough water to fully cover the eggs, once added. Place the saucepot over high heat and add the vinegar. Bring to a boil. (The vinegar makes the eggs easier to peel.)
Use a needle to prick a hole in the wide end of each egg. (This does two things, it helps the egg to form a prettier shape as it cooks, without a flat end, and it makes the eggs easier to peel.)
Once boiling, use a skimmer or slotted spoon to gently lower the eggs into the water. Once they are all in, set a timer for 6 or 7 minutes, depending on wether you want saucy yolks or custardy yolks.
Meanwhile, fill a medium mixing bowl with ice and cold water. The moment the timer goes off, use the slotted spoon to carefully move the eggs from the boiling water to the ice water to cool. Cool for at least 10 minutes.
While the eggs are cooling, pull out a large jar or measuring pitcher, that will fit all the eggs. In the container, mix the soy sauce, mirin, and brown sugar.
Once the eggs have cooled, gently peel off the shells, and lower them into the marinade. Chill the eggs in the marinade for 1-4 hours. Make sure to gently stir them at least once to reduce discoloration spots.
When ready to serve, remove from the marinade, and cut in half.
Ramen eggs can be used immediately and do not have to be marinated. However, if you have time, it’s worth the effort.Marinated ramen eggs will keep well for up to 4 days in the refrigerator. Remove from the marinade and transfer to an airtight container before placing in the fridge.If you don't marinate the soft-boiled eggs, however, they need to eaten within a day or so. Keep in the fridge and enjoy within 48 hours. They'll be heated to perfection once they are added to the hot soup.