Cacio e pepe with egg yolk

Put a pot of water on for the spaghetti, and cook the spaghetti for half the time indicated on the package. Meanwhile, coarsely crush the peppercorns in a mortar. Toast the pepper in a pan until well roasted. Add a small lump of butter to the pan with the pepper, and let it melt. Take some of the cooking water from the spaghetti and pour it over the pepper, letting it simmer for a couple of minutes. When the spaghetti has cooked for half the time, take it out and add it to the pan. Now it should finish cooking in the pan, with cooking water from the spaghetti pot being added regularly. Let the water evaporate before adding more cooking water (as with a risotto). When the spaghetti is al dente, take the pan off the heat, and add the egg yolk, stirring constantly to avoid it turning into scrambled eggs. Now the dish should be nice and creamy. If it becomes a bit too dry, you can add a little cooking water to achieve the desired consistency.