Steps

1. Prepare boiling water for the eggs. At this time, take out 30g of butter to soften at room temperature for spreading on the bread.
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2. Once the water is boiling, gently add the 4 eggs and boil for 8 minutes. [Key Tip!] Lightly rinse the eggs with water before adding to the boiling water and use a ladle or tongs to gently place them in to prevent cracking.
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3. Remove the boiled eggs from the heat and immediately plunge them into ice water to cool thoroughly. [Key Tip!] Rapidly cooling in ice water creates a gap between the shell and the egg white, making it easier to peel.
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4. For the cooled eggs, run a little water over them and gently tap all over to create small cracks in the shell. Once cracked, peel them smoothly under running water.
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5. In a large bowl, separate the yolks from the whites of the peeled eggs and place the yolks into the bowl. Mash the yolks thoroughly with a rubber spatula or spoon. [Key Tip!] Mashing the yolks well ensures they are evenly distributed, creating a rich egg sauce.
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6. Add 3 tbsp mayonnaise, 1 tsp lemon juice, 1 tsp sugar, and 2 pinches of salt to the mashed yolks. Mix well until the mixture becomes thick and creamy.
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7. Finely crumble the cooked egg whites by hand and add them to the egg sauce. [Key Tip!] Add the egg whites last, leaving some texture, to enjoy a contrast in the sandwich.
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8. Cream the softened butter using a spoon or rubber spatula until it forms a paste. [Key Tip!] Spreading butter on the bread prevents the moisture from the filling from soaking into the bread and making it soggy.
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9. Add 4g of Japanese mustard to the creamed butter and mix thoroughly.
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10. Spread the prepared mustard butter evenly to the edges on all 4 slices of thick-cut bread. [Key Tip!] Spreading generously on both sides enhances the buttery flavor and prevents the bread from becoming soggy due to the filling's moisture.
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11. Spread the egg sauce on 2 of the buttered bread slices, then top with the other 2 slices to make sandwiches.
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12. Wrap the sandwiches tightly in plastic wrap and refrigerate for about 15 minutes to allow the flavors to meld. [Key Tip!] Wrapping tightly prevents air from getting in, which helps the bread and filling meld together for a better eating experience.
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13. Cut the sandwiches while still wrapped in plastic wrap and arrange them on a plate. [Key Tip!] Cutting the soft egg sandwich with the plastic wrap on ensures it won't shift and results in a neat presentation.
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14. For bento boxes, boil the eggs for 12 minutes to ensure they are fully cooked, reduce the mayonnaise slightly, and adjust the salt to taste.
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