1. Fill your egg boiler with water and add the eggs. Remove 3 eggs after 7 minutes and immediately transfer them to a bowl of ice water. Allow the remaining eggs to continue boiling for 12 minutes.
2. While the eggs are boiling, in a separate bowl, combine the egg yolks, salt, pepper, and other seasonings. Using a hand blender, whisk the mixture while slowly pouring in the oil to achieve a creamy mayo consistency. Once done, transfer the homemade mayo to an airtight jar or container.
3. In a bowl, mash the hard-boiled eggs along with chives, homemade mayo, mustard sauce, salt, and pepper. Mix well until all the ingredients are thoroughly combined.
4. Take slices of milk bread and trim off the edges. Spread the egg filling generously between two slices of bread and add a whole boiled egg that was set aside.
5. Press the slices together to create a sandwich. Cut the sandwich into diagonal halves.
6. Serve & enjoy the delightful Japanese Egg Sandwich!
Note: Feel free to adjust the seasoning and mayo consistency according to your taste preferences. The sandwich can be served chilled or at room temperature.