Begin by placing the eggs in a pot filled with cold water. Bring the water to a boil over medium-high heat. Once boiling, reduce the heat to a simmer and cook the eggs for 9 to 10 minutes. After the time is up, transfer the eggs to an ice bath for 5 minutes.
Once the eggs are cool, peel them carefully under running water if needed. In a medium bowl, use a fork to mash the peeled eggs until they reach a creamy consistency.
Stir in the mayonnaise, soy sauce, and mirin into the mashed eggs. Season with salt and pepper to taste, mix thoroughly until all ingredients are well incorporated.
For an extra layer of flavor, optionally spread a thin layer of mustard on one side of each slice of bread.
Evenly divide the creamy egg mixture between two slices of bread, spreading it to the edges. Place the remaining two slices of bread on top.
For a polished look, carefully trim the crusts off the sandwiches using a sharp knife.
Using the knife, cut each sandwich into halves or quarters, adjusting based on your desire for serving size.
For a finishing touch, sprinkle the tops with finely chopped chives.
Notes
Serve on a beautiful platter, and consider placing a small bowl of pickles or chips on the side for additional texture and flavor.