Place large-sized eggs in a pot of cold water, ensuring they’re submerged. Bring the water to a rolling boil over medium-high heat, then cover and reduce the heat to medium-low. For medium-boiled eggs, cook for 7 minutes and 30 seconds, and for hard-boiled, 9 minutes. Once done, transfer the eggs immediately to an ice bath for 10 minutes before peeling.

While the eggs cool, mash the hard-boiled eggs in a mixing bowl until chunky. Add in whole milk, Kewpie mayonnaise, sugar, salt, and black pepper, mixing thoroughly. Taste and adjust seasoning. Cover and refrigerate for 10 minutes.

Spread unsalted butter generously on one side of each slice of shokupan. Cut the cooled medium-boiled eggs in half and place them cut side up on two slices of bread. Top with the chilled egg filling, spreading evenly. Cover with buttered bread slices, butter side down.

Slice the assembled sandwich in half diagonally and serve immediately. Enjoy your Tamago Sando on its own or with a side of pickled vegetables.