Bring a large pot of water to a boil and add salt until it tastes like the sea.
Put guanciale and 2 tsp. olive oil into a skillet or Dutch oven that's large enough to hold all the pasta. Cook until it's lightly crisped and most of the fat has rendered out, 9 to 12 minutes. Take skillet off the heat and spoon off fat except for about 1 tbsp.
When water is boiling, add pasta and cook according to package directions until almost al dente. When pasta is almost ready, add snap peas to pasta pot.
Put skillet back over medium heat to reheat guanciale gently. With a ladle or a measuring cup, scoop out about 1/2 cup of pasta cooking water. Drain pasta and snap peas and add them to skillet along with cacio e pepe butter. Toss well to incorporate, adding a few drops of pasta water in order to make a cloaking, creamy sauce. Taste and adjust with more salt or pepper, though you probably won't need any.
Transfer to serving bowls and top with grated pecorino. Serve right away.