Preheat the oven to 190° / 375°F.
Peel the potatoes and cut them into 1-inch quarters.
Place them in a pot and top with water.
Bring to the boil and add a teaspoon of sea salt.
Cook on medium for about 15 minutes until the potatoes are soft.
Drain.
Add the potato pieces to a ricer a bit at a time over a medium-sized bowl.
Top the still warm potato mixture with butter and whisk.
Once the butter is melted, add milk, and season to taste.
Allow the potatoes to cool slightly.
Add the three yolks and whisk again (to combine).
Spoon the mixture into a piping bag.
Pipe the duchess potatoes in three circular layers on a lined baking sheet, resembling rosettes.
Beat the single egg and brush the rosettes carefully with egg wash.
Bake until golden brown (about 20-25 minutes).
Top with chopped chives or parsley, and serve.