Put guanciale in a large skillet and place over medium-low heat. Cook, stirring frequently, until fat renders but guanciale is not browned, about 5 minutes. Transfer the guanciale to your serving bowl; reserve the drippings.
Add egg yolks and egg to your serving bowl; whisk to blend.
Meanwhile, cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente. Drain, reserving around a 1/2 cup pasta cooking liquid.
To egg mixture, immediately add spaghetti, 2 tablespoons pasta cooking liquid, and 1 teaspoon guanciale drippings; toss to coat. Working in 3 batches, gradually add Pecorino, stirring and tossing to melt between batches. Add lots of black pepper (around 2 teaspoons). Toss until the sauce thickens, adding more pasta water by tablespoonfuls if needed. Season to taste with salt and black pepper.
Divide among bowls (or don't). Garnish with Pecorino.