Render pancetta in a pan and remove.
Clean the leeks and slice them thinly. Melt the butter in a large pan with a drizzle of oil, then add the prepared leeks. Season with salt, stir, and cover. Let the leeks simmer over low heat until tender, about 25 minutes. Keep the mixture moist by adding hot water as needed.
Once tender, transfer the leeks to a bowl, add the egg yolks and a ladle of boiling water, and blend. Add about half the grated Parmesan cheese and a little cream, then blend again.
Transfer the resulting cream to the pan used previously, add the remaining cream, season with salt, and heat.
Cook the pasta in boiling salted water.
Drain the pasta al dente and add it to the pan, thinning the sauce with a little of the cooking water if necessary. Stir and let it cook for a few minutes, then finish by adding the remaining grated Parmesan, pancetta, and ground pepper.
Serve the spaghetti with leek cream, adding grated Parmesan and ground pepper to each serving.