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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 for 20 minutes, check with a fork if they're done. Make a puree with the potatoes, be sure it's a smooth puree without lumps.
You can use a fine sieve for this by running the potatoes trough using a big ladle. MIX in the butter and egg yolks.
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.
Lay a baking tray with greaseproof baking 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!