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A classic French cuisine item, which name is actually "pommes de terre duchesse". Most people know it by the exact name of this recipe.
Peel the potatoes and cut them into big chunks. Boil the potatoes for 20 minutes, check with a fork if they're done. Create a mash of the potatoes, be sure it's a smooth puree without lumps.
You can use a fine sieve for this by running the potatoes through using a big ladle. Mix in the butter and egg yolk.
Stir it all together and season with salt, pepper and nutmeg. Scoop the puree in a piping bag, be sure you're using one with a nozzle.
Line a baking tray with parchment paper. Pipe rosettes on the baking tray. Bake the pommes duchesse for about 10 to 15 minutes at 200 degrees Celsius or 390 degrees Fahrenheit.
The cooking time depends on how powerful your oven is. Enjoy!