Preheat the oven to 220ºC.
Wash and scrub the potatoes with a vegetable brush. Dry and prick each one with a fork and arrange on a baking tray. Sprinkle some olive oil, salt and pepper over and bake the potatoes for about an hour in a preheated oven until soft and cooked through.
Let the potatoes cool then halve them, spoon the soft flesh from the skin and mash it until fine. (Press the fine potatoes through a sieve for a soft, fluffy result.) Mix the mashed potatoes with the yolks and enough flour to form a soft dough. Season well with salt, pepper and nutmeg.
Divide the dough into 4 equal parts and roll each portion out into a thin sausage of about 30cm long and 1.5cm thick on a floured surface. Using a sharp knife, cut into 2cm portions. Roll the bite-sized portions in semolina and lightly press flat with a fork. Arrange on a tray and refrigerate for about 20 minutes.
Bring a large pot of salt water to the boil and cook the gnocchi in batches for about 3 minutes at a time until they float to the surface. Remove from the water with a slotted spoon and keep warm. Repeat until all the gnocchi is cooked. Serve tossed in melted butter, sage, roasted pine nuts, parmesan shavings and a sprinkling of truffle salt.
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