Cook the Eggs

1. Boil the Eggs: Start by boiling the eggs. Place them in a pot of cold water, bring it to a boil, then reduce the heat and let them simmer for about 10 minutes for hard-boiled eggs. Once done, transfer the eggs to an ice bath or run them under cold water to cool.
2. Peel and Mash: Once the eggs are cool, peel them and chop them roughly. Then, mash them with a fork until they are broken into small, uneven pieces.

Prepare the Filling

3. Mix the Filling: In a mixing bowl, combine the mashed eggs with Japanese mayo, butter, mustard (if using), salt, and pepper. Stir everything together until the mixture is smooth and creamy. Taste and adjust the seasoning to your liking.

Assemble the Sandwich

4. Prepare the Bread: If desired, trim the crusts off the bread to give it that soft, clean look (this is optional but often done in Japan for a more refined presentation).
5. Spread the Filling: Generously spread the egg mixture onto two slices of bread, making sure to cover the entire surface.
6. Close the Sandwich: Place the remaining two slices of bread on top and press gently to seal.