Prepare a steaming rack over a pan of simmering water. Alternatively, bring 3cm of water to a boil in a pan.
Place the eggs on the steaming rack, or in the shallow layer of water. Cover the pan and steam the eggs for 12 minutes.
Remove from the pan and allow the eggs to cool naturally. Peel them.
Chop the eggs up finely, and mix in a bowl with the mayonnaise, salt and sugar.
If using butter, spread a thin layer on one side of each slice of bread.
Sandwich the egg mayonnaise between the bread, buttered side facing in.
Cut the crusts off for a proper Japanese look, and cut in half. Serve!