You can of course buy them ready-made, but if you want to cook them the same way, this is how to do it:
Remove the peel from the potatoes and cook them in large pieces in salted water, then press them through a potato ricer or mash them. Alternatively, use the appropriate disc of a food processor, but never grate the potatoes! This will make the mixture too fine and give it a soapy consistency.
Stir the potato mixture and half of the butter with a whisk. Bring milk or whipping cream to a boil and stir into the mixture until the mashed potatoes have the right consistency. Season to taste with salt, pepper and a pinch of nutmeg. Stir 1 egg yolk and the remaining butter into the potato mixture, fill the mixture into piping bags, place small heaps on a buttered baking tray, brush with beaten egg yolk, bake at 225 °C until golden brown.