On a clean surface, form a well with the flour. Add the egg yolks to the center and mix with a fork. Once it comes together, knead by hand until combined. Cover and let rest for 10 minutes. Knead again until smooth and rest for 30 minutes.
In a bowl, mix ricotta, Parmigiano Reggiano, nutmeg, and salt until smooth. Transfer to a piping bag and set aside or refrigerate until ready to use.
Roll out the dough using a pasta machine, starting from the widest setting and working down to the second-to-last setting.
Pipe a circle of the ricotta filling (like a well) for the egg yolk to sit in the center.
Lay another pasta sheet over the filling. Press gently to seal the edges, making sure there are no air pockets. Use a round cutter to shape and cut the raviolo.
Bring a pot of salted water to a gentle boil. Carefully lower the raviolo in and cook for about 2 minutes, until the pasta is tender but the yolk remains runny.
In a cold pan, add butter and turn heat to low. Add a tablespoon of water at a time and stir continuously. This sauce should be luscious and coat the back of a spoon. You can add fresh sage leaves at this stage if desired.
Plate the raviolo, drizzle with butter, and enjoy immediately.