Bring water to a boil. To make sure the cooking water will be very starchy, use half, or even less, of the amount of water you would normally use for cooking bucatini.
In the meantime, add half of the grated Pecorino romano in a bowl.
When the water is boiling, add some salt to the water. Then add bucatini and cook very much al dente, leaving them a bit raw.
When the bucatini are ready, drain them over a large bowl to save all the cooking water. Set the bucatini aside.
Toast ground black pepper on a large pan over medium-low heat. Then add ⅔ of the pasta cooking water to the pan.
Add the bucatini into the pan with the pepper and water to finish cooking them. This will take only a few minutes. Mix the well and add some more cooking water if the pan is too dry.
In the meantime, add one ladle of the cooking water into the bowl of Pecorino romano and whisk well to create a cream. If needed, add a bit more cooking water.
Once you have a creamy texture, whisk in the rest of the Pecorino (save a small amount). Finally, if needed, add more cooking water to create a creamy texture without lumps.
Once the pasta is cooked, add the cream into the pan, take off the heat and mix extremely well. Then add the remaining small amount of Pecorino, mix, and serve immediately.
Notes
As pecorino cheese is very salty by itself, play it safe with the amount of salt in the pasta cooking water.
Do not toast for too long. A couple of minutes is ok.
When boiling water for the pasta, add less water than you usually would.
When stirring the pecorino with the starchy water you put aside, do not get discouraged if the cheese is clumpy. The more you mix, the more it will emulsify. Adding just a bit more cooking water might also help.