Bring a small saucepan with enough water to cover the egg to a rolling boil. Place cold egg into water and bring back to a rapid boil and cook like so for 30 seconds. Lower heat to simmer for 7 minutes, then transfer egg to an ice bath to halt cooking. Once cool enough to handle, peel and set aside.

Toast bread until golden. Transfer to plates and slather with mayo. Layer prosciutto slices on top, then use an egg slicer for evenly thin egg slices and arrange them on the prosciutto. Scatter the green and chives, and season with salt and pepper.