Preheat the oven to 425°F (220°C).
Scrub the potatoes if dirty, then put them in a large saucepan, cover with cool water and add a big pinch of salt. Place over high heat, bring to a boil and cook until a knife slides through them without any resistance, 15 to 25 minutes. (Don’t let them go too far, as you don’t want to be serving mashed potatoes!)
Meanwhile, line a plate with paper towel. Toss the pancetta into a large skillet along with a splash of the oil. Set over medium-high heat and fry until crispy, 5 to 7 minutes. Using a slotted spoon, transfer the pancetta to the lined plate to drain. Reserve the fat.
Line a rimmed baking sheet with foil. Drain the potatoes in a colander, then add them to the baking sheet. Use a fork or spatula to gently press down on them until the skin bursts and they flatten.
Drizzle with the reserved pancetta fat plus 1 to 2 tablespoons of the olive oil and season well with salt and pepper. Roast until the potatoes begin to brown and crisp up, 20 to 30 minutes.
Turn the broiler to high, slide the potatoes under the broiler and cook until golden brown and crispy, 5 to 10 minutes more. Keep an eye on them as they can burn quickly.