Place a large pot of water with salt over high heat. Bring to a boil and toss in the spaghetti to cook for 8 minutes.
Place the egg yolk in a small bowl, add the black pepper, half the pecorino cheese and 1 tablespoon of water. Whisk into a creamy consistency and set aside to use later.
In the meantime, a couple of minutes before the pasta is cooked; place a pan over medium heat, toss in the guanciale and cook for 2 minutes until they're golden brown.
Once the pasta is cooked, turn the heat off from the pan of guanciale, then take the pasta from the boiling water and transfer it into the pan.
Pour the creamy egg yolk mixture into the pan and mix until it is all incorporated. Also, add a tablespoon of pasta water. Toss together and serve immediately. Top with the rest of the pecorino and some black pepper
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Notes
Adding water to the egg-cream mixture prevents it from scrambling. Removing the pan from the heat before adding the egg is also important.