Fill a pasta pot halfway full with water, season with the kosher salt, and bring to a boil. Cook the pasta to al dente, usually a couple of minutes shy of what the box says. You want the pasta to have a little bite, but be cooked all the way through. Reserve 1 1/2 cups of the pasta water in a measuring cup and then drain the pasta in a colander.

In a large skillet, melt the butter and olive oil over medium heat. Once the butter is melted, add the pepper and cook for 1 minute (this helps to wake up the flavors). Add 2/3 cup of the pasta water to the pan and simmer for 2 minutes or so to reduce slightly. When you drag your spoon across the bottom of the pan, you should see the pan exposed and the water shouldn't run immediately back together.

Add the pasta to the pan and stir to coat, if the pan is dry add a little more water. Add the parmesan and egg yolk and stir really well to melt the cheese and disperse the egg. If the pasta is dry, add a little more pasta water. The sauce should be very creamy at this point.

Add salt to your taste, if necessary but there is lots of salt in the parmesan and the pasta water so keep that in mind before adding more. Serve topped with more parmesan and pepper if desired.