To serve 4 people, cook 1kg of floury potatoes in a large saucepan of boiling salted water for 20 minutes, or until very soft.
Drain through a colander and leave for a few minutes to allow the excess moisture to steam off. Mash the potatoes until very smooth - a mouli-legumes or potato ricer is best for this - making sure there are no lumps. Place in a large mixing bowl and leave to cool.
When the potato is cold, mix in 200g pasta flour, then beat in 2 egg yolks to form a soft dough.
Divide the dough into quarters and roll each piece into a long 'sausage' shape about 2cm in diameter. Cut each 'sausage' into 2cm lengths and place on a floured board.
Slightly flatten each piece with the back of a fork, leaving fork marks on one side - these grooves will catch any accompanying sauce when you plate up.
Bring a large saucepan of generously salted water to the boil. Tip the gnocchi into the pan of boiling water and cook for 2-3 minutes, until they float to the top. Remove with a slotted spoon and serve straight away with a sauce, preferably tomato.