In a small saucepan, combine all the ingredients for the marinade.
Bring it to a boil and whisk it a few times to let the sugar dissolve completely. Once boiling, lower the heat and simmer for 1 minute. Turn off the heat. Set aside to cool completely.
To Make the Soft-Boiled Eggs
Add 4 cups (1L) water (plus 1 Tbsp vinegar and 1 tsp salt—optional, but it helps with the peeling of the eggshells) to a medium saucepan and bring it to a boil. There should be enough water to cover the eggs by at least 1 inch (2.5 cm). Once fully boiling (not a gentle, but a full boil), take out the eggs from the refrigerator and carefully and gently lower one egg at a time into the boiling water with a mesh strainer/skimmer or a ladle. When you add the first egg, set a 7-minute timer. You can cook them 6 to 6½ minutes for a runny egg yolk and 8 to 9 minutes for a custard-like egg yolk.
Lower the heat to maintain a gentle boil. Make sure the water is simmering, but not bubbling so strong that the eggs bounce around. If you want your egg yolks to be in the center, gently rotate the eggs with chopsticks once in a while for the first 3 minutes.
After 7 minutes, immediately take out the eggs and shock them in an iced water for 15 minutes.
Once the eggs are completely cool, gently crack the wide bottom end of the egg and start peeling the shell vertically toward the pointy top. Dip the egg in the iced water a few times to help with the peeling. Once you peel one section vertically, the rest of the shell comes off easily.
To Marinate the Eggs
Place the eggs in a plastic bag and add the marinade to the bag. Why am I recommending a plastic bag? With a plastic bag, we don't have to use a lot of marinade to submerge the eggs. This marinade is used only one time for food safety reasons, so it’s most economical to prepare no more than the amount you need. If you use a container instead of a bag, it requires more marinade to submerge the eggs.
Remove the air from the bag and use a clip or rubber band to seal the bag right above the eggs. This way, the eggs are completely submerged in the marinade. Refrigerate for 8 hours or overnight. You can rotate the eggs occasionally if you like.
Remove the eggs from the marinade and cut them in half lengthwise to serve. Use a piece of string, fishing line, or cheese cutter (that's what I used here) to cut the eggs in half cleanly. Enjoy the eggs as a snack, in bento, or as a ramen topping. If you want to warm up the eggs, soak the bag in warm water to bring up the temperature faster.
Keep the eggs refrigerated at all times. Do not freeze the eggs as they change the texture (except for the Tamagoyaki recipe). The ramen eggs will get saltier so take them out from the marinade after 12-24 hours (depending on your preference). Enjoy the ramen eggs within 3-4 days if your eggs are soft-boiled. If your eggs are hard-boiled, you can keep them in the refrigerator for up to a week. For food safety reasons, I recommend not reusing this marinade with new boiled eggs. You can repurpose this marinade as a seasoning sauce for your stir-fried or simmered dishes but use it soon.