Begin by hard-boiling four eggs in a pot of water for exactly 10 minutes. Transfer the eggs to an ice bath for a few minutes to stop the cooking process.

Peel the cooled eggs and mash them in a mixing bowl. Mix in Japanese mayonnaise and sugar, stirring until creamy. Adjust sweetness as desired.

Take two slices of soft bread and spread the creamy egg mixture evenly across one slice. Top with the second slice and press down gently.

Slice the sandwich in half diagonally and serve immediately, ideally with matcha or iced tea.