Place the eggs in a pot of boiling water and cook for about 10 minutes. Transfer to an ice bath to cool, then peel the shells.
Separate the egg whites and egg yolk. Chop the egg whites.
In a bowl, add the egg yolks, sugar, Dijon mustard, Kewpie mayonnaise, salt, and pepper.
Mix until well combined. Adjust seasoning to taste.
Add the egg whites and mix.
Spread a thin layer of salted butter on each slice of bread.
Generously spoon the egg mixture onto one slice of bread, then top with the second slice.
Slice the sandwich in half for a clean, classic presentation.