Japanese Egg Sandwich (Tamago Sando)

To make the egg salad

1. Prepare an ice bath in a big bowl.
2. Bring a medium-sized pot of water to boil. The water level should be high enough to cover the eggs. Once the water is boiling, carefully lower the eggs using a ladle. Boil for 7 minutes (*Footnote 2), then turn off the heat and let the eggs sit for 1 minute for medium soft boiled eggs. Boil for 10 minutes for hard boiled eggs. Once done, immediately transfer the eggs to the ice bath and let sit for 2 minutes to stop cooking, but remove them while they’re still lukewarm. Then peel the eggs.
3. Transfer the peeled eggs to a large bowl. Mash the eggs with a fork a few times to break them up. Add the sugar, salt and pepper. Keep mashing the eggs into small pieces that are smaller than a pea but larger than minced. You can use a paring knife to assist in the mashing, which might make it easier.
4. Add the mayonnaise (and milk, if you are using hard boiled eggs). Mix everything together. Taste the egg salad and add more salt, pepper, or mayonnaise if needed.

To cook and assemble the sandwich

1. To assemble one sandwich, spread 1/2 tablespoon of the butter evenly onto each slice of bread. Evenly spread the egg salad onto one slice of bread. Put the other slice of bread on top, buttered side down. Gently press the bread down. Then slice the crust off the bread. Assemble the other sandwich using the same method. Garnish with chives, if using.
2. Cut the sandwiches in half and serve immediately.

To Store

1. To store the sandwiches, wrap up each sandwich with plastic wrap and store it in the fridge for up to 2 days.