Place the eggs in a pot, add just enough cold water so that they are fully covered, and put on high heat. Add a little salt to the pot as salt makes them easier to peel.
Set on high heat to bring to boil. As soon as the water is boiling fast and hard, turn the heat off and cover the saucepan. Leave the eggs in the covered pan for 11 minutes.
Drain the cooked eggs and run them under cold water until they have fully cooled (this will stop the eggs from cooking and will make them easier to peel). Alternatively, place them in a bowl of water with ice for 5 mins.
Peel the eggs and slice them in half from top to bottom.
Using a small spoon, scoop out the cooked egg yolks and put them into a bowl. Add the yogurt, mustard, and a big pinch of salt. Mash the ingredients until they are smooth and have come together. If you prefer a smoother, fluffier texture, use a food processor.
Either spoon the egg mixture into the cavity in the egg white, or for more accuracy and presentation, place the egg mixture into a food bag, snip the corner off with scissors and pipe into the egg cavity.