Bring a large pot of water to a boil over medium-high heat. Meanwhile, finely grate 1 ounce Pecorino Romano cheese on a Microplane (2/3 to 1 scant cup). Cut the guanciale into 1/2-inch-wide by 3/4- to 1-inch-long pieces (about 1 1/2 cups).
Place the guanciale in a large skillet, preferably nonstick. Cook over medium heat until it starts sizzling, about 2 minutes. Reduce the heat to medium-low and cook, stirring occasionally, until the guanciale is browned and crisp, 6 to 10 minutes more.
Meanwhile, add 1 pound dry spaghetti and 2 teaspoons kosher salt to the boiling water. Cook according to package directions until the pasta is just al dente, 8 to 12 minutes. Place 4 large eggs and 2 large egg yolks in a medium bowl and whisk until combined. Add the Pecorino and a pinch of kosher salt, and whisk until combined.
When the guanciale is ready, transfer to a plate with a slotted spoon. Add 1 teaspoon coarsely ground black pepper to the skillet, turn off the heat, and stir to combine. Let cool for 1 minute.
While whisking constantly, slowly stream in 1 tablespoon of the fat from the skillet into the egg mixture, then slowly stream in 1/4 cup of the pasta cooking water.
When the spaghetti is ready, turn off the heat, then transfer it directly to the skillet with tongs. It’s OK if some water clings to the pasta. Add 1/4 cup pasta water to the skillet and toss until the pasta is well-coated and most of the water and fat in the pan have been absorbed.
While tossing the pasta constantly, slowly start pouring the egg mixture into the skillet, pouring into the center over the pasta and not around the edges.
Return the guanciale to the pan. Turn the heat on to the lowest setting and cook, tossing constantly, until the sauce starts to thicken and coats the pasta, 1 to 2 minutes. Make sure to move the skillet around so any hot spots on the burner don’t scramble the eggs. Remove the pan from the heat.
Taste and season with more kosher salt and black pepper as needed. Transfer to a large bowl or individual serving bowls and serve immediately with more grated Pecorino and a few grinds of pepper on top if desired.