Tamago Sando

Place the eggs in a medium saucepan and cover with cold water by 1 inch. Bring to a boil over high heat. Once boiling, remove from heat, cover, and let sit for 10 minutes. The eggs should be fully cooked with firm yolks.
Drain the hot water and immediately transfer the eggs to a bowl of ice water. Let cool for 5 minutes, then peel and discard the shells.
In a medium bowl, finely chop the peeled eggs using a fork or pastry cutter. The mixture should be slightly chunky but well combined.
Add the mayonnaise, Dijon mustard, sugar, salt, and black pepper to the chopped eggs. Stir until evenly mixed and creamy.
Spread the softened butter evenly on one side of each slice of bread. This will create a crisp outer layer when assembled.
Evenly distribute the egg mixture onto the unbuttered side of two slices of bread. Spread it to the edges for full coverage.
Top each with the remaining bread slices, buttered side facing out, to form two sandwiches. Press gently to seal.
Using a sharp knife, carefully trim the crusts off the sandwiches, then cut each sandwich diagonally into two triangles. Serve immediately or wrap tightly for later.