Decadent, rich and well worth the effort. Guanciale is traditionally used in carbonara and if you can find it, use it instead of pancetta, it's a worthy splurge. Keep the egg whites for meringues, or to make scrambled whites the next day as penance for your unbridled potato joy.
Put the potatoes in large saucepan, cover with cold water by about 1 inch and add 2 tablespoons salt. Bring to a boil, uncovered, over high heat, and then reduce the heat to medium-high and simmer briskly until fork tender, about 45 minutes. Drain the potatoes, reserving 3/4 cup of the water. Quarter the potatoes and pass them through a food mill or ricer back into the pan. (Peel the quarters first if you are using a ricer).
Fold in 1/4 cup reserved pancetta fat, pancetta bits, Pecorino and reserved cooking water into the potatoes. Season with salt. Fold in 4 egg yolks until well blended. Spoon or pipe six 1-cup mounds onto the prepared baking sheet and top with pepper. Make little wells in each mound with the back of a spoon. Bake until the edges are golden brown, about 15 minutes. Remove from the oven, fill each well with one of the remaining 6 yolks and place back in the oven to lightly cook the yolk, 2 to 3 minutes. Serve immediately.