Preparation

• Preheat the oven to 230°C. Place the potatoes on a baking tray and sprinkle with salt. Bake until they are completely soft, about 45 minutes.
• Cut in half lengthwise (be careful, they're hot!) and use a spoon to remove the flesh. Press through a potato ricer, spread evenly on a clean work surface and leave to cool.
• Drizzle the egg yolk over the top. Sift two thirds of the flour evenly over the potatoes. Using a dough scraper, chop through several times, as for making shortcrust pastry, then shape the mixture into a loose ball.
• Press onto the work surface, sprinkle with the remaining flour, and gently knead until an even dough forms. Dust the dough with flour.
• Using the dough scraper, cut off a portion of dough about 5cm thick and roll into a cylinder about 1.5cm thick on a floured work surface; use light pressure when rolling to prevent sticking. 
• Cut into 3cm long gnocchi with the dough scraper. Repeat with the remaining gnocchi dough until all is used up, dusting well with flour as needed.
• Bring a large pot of very well salted water to the boil.
• Meanwhile, melt the butter in a large frying pan over a medium heat until it foams. Add the sage leaves and fry until aromatic and the butter begins to brown slightly, then remove from the heat.
• Add the gnocchi to the boiling water in batches. Stir once very gently so they don't stick. When they float to the surface, wait about 20 seconds and then lift them out with a slotted spoon and transfer to the pan with the butter and sage, taking some of the cooking water with the gnocchi.
• Once all are added to the pan, toss the gnocchi in the sage butter over medium heat, adding a splash of cooking water if needed, until the gnocchi are coated in a rich, creamy sauce, about 1 minute.
• Carefully transfer to plates, sprinkle with grated Parmesan and serve.