Steps

1. Gently place 3 eggs into boiling water, then reduce heat and boil for 12-13 minutes. [The Secret Here!] Boil until the yolks are cooked through.
   🕒 01:25
2. Immediately turn off the heat and transfer the boiled eggs to cold water.
   🕒 01:46
3. In the cold water, tap the eggs on the bottom of the bowl to crack the shells all over, then peel them under water.
   🕒 01:49
4. Make an incision in the boiled eggs, then open them up and separate the yolks and whites.
   🕒 02:31
5. Thinly slice the egg whites. [The Secret Here!] Instead of finely chopping, slicing thinly creates a silky texture.
   🕒 02:54
6. In a separate bowl, mash the yolks with a rubber spatula. Add 1 tbsp of milk and mash until smooth.
   🕒 03:28
7. Add 2-3 tbsp of mayonnaise and 1 pinch of sugar to the yolk bowl and mix.
   🕒 03:48
8. Mix until the yolks become a fluffy cream, incorporating plenty of air. [The Secret Here!] This step determines the fluffiness of the egg spread.
   🕒 04:07
9. Add the sliced egg whites to the yolk bowl. Season with salt to taste and mix.
   🕒 04:35
10. Make the mustard butter by mixing 10g of butter (approx.), softened at room temperature or heated for 10 seconds in the microwave, with 1/2 tsp (approx.) of Japanese mustard paste. [The Secret Here!] Generously apply the mustard butter.
    🕒 05:09
11. Trim the crusts off 2 slices of shokupan (8-slice bread).
    🕒 05:41
12. Generously spread the mustard butter on both sides of the bread, enough to cover the white part.
    🕒 06:16
13. Place one slice of bread on plastic wrap, and gently pile the egg spread onto it, letting gravity do the work. [The Secret Here!] Do not press down on the egg spread; piling it loosely preserves its fluffiness.
    🕒 06:32
14. Gently place the other slice of bread on top without crushing the filling, then wrap with plastic wrap.
    🕒 06:58
15. Refrigerate the wrapped egg sandwich to allow it to set.
    🕒 07:18
16. Remove from the refrigerator and cut in half with a knife.
    🕒 07:26