Cook pasta in boiling salted water until al dente (about 9 minutes).
While pasta cooks, whisk egg and egg yolk in a mixing bowl then stir in the cheese, salt and black pepper.
Also start browning the pancetta and garlic in butter in a large skillet so that the fat is rendered but not to the point where it is fully crispy; keep it a little softer; push the pan off the heat while the pasta finishes cooking.
When the pasta is done, siphon off a cup or two of cooking water to a bowl and set aside, then drain the pasta.
Once drained, toss the pasta with the egg/cheese mixture, until fully coated.
Put the skillet back on the heat on low, and add the pasta, tossing again along with enough of the saved cooking water to make the sauce creamy but not watery. The egg will cook as you mix.
Serve garnished with a little chopped parsley and extra cheese if you like.