Boil the eggs: Place the eggs in a saucepan and add enough water to cover them. Bring to a boil over medium heat, then reduce the heat and simmer for 10 minutes. Transfer to an ice bath.
Make the egg salad: Peel the eggs and mash them in a bowl with a fork or masher. Add mayonnaise, yogurt, vinegar, salt, and pepper, then mix well.
Assemble the sandwich: Spread mustard and mayonnaise on the bread. Add the egg salad, then top with another slice of bread and gently press.
Serve: Slice in half and enjoy!
Notes
Adjust the mayo to your taste. Add more for extra creaminess or less for a lighter feel.
For cleaner slices, wrap the sandwich in plastic wrap before cutting, then slice through the wrap to help it hold its shape.
And for that classic Japanese convenience store look, trim the edges!