Preheat the oven to 180°C. Place the potatoes on a baking tray and bake in the oven for 70–90 minutes until tender, depending on their size. Remove from the oven, cut in half, scoop out the flesh with a spoon and discard the skins. Pass the potato through a drum sieve and weigh out 550 g potato purée; place in a bowl, add the flour, Parmesan cheese, egg yolks, chopped truffle and the truffle oil and work into a dough. Season with salt and freshly ground pepper. Turn out the dough onto a lightly floured work surface and knead for 3 minutes. If it feels wet, add a little more flour. The dough should be firm yet soft. Spoon the dough into a piping bag fitted with a 3 cm. plain tube and pipe out in lines onto a lightly floured surface. Cut the gnocchi into 3 cm. lengths.
Bring a large saucepan of salted water to the boil and cook the gnocchi in several batches. Drop the gnocchi into the water; they will float to the surface within 3–4 minutes when cooked. Lift out of the water and plunge into iced water to refresh. Drain and dry on a clean kitchen cloth. If not using immediately, mix through a little extra virgin olive oil to prevent the gnocchi from sticking together. Cover and refrigerate until ready to use.