In a frying pan over medium heat, cook the pancetta, stirring occasionally until crispy, 4 to 5 minutes. Transfer to paper towels to drain.
In a saucepan over medium-high heat, melt 2 tablespoons of the butter. When it foams, add the breadcrumbs and cook, stirring, until golden brown, about 5 minutes. Transfer to a plate and set aside.
In a large pot, melt the remaining 2 tablespoons of the butter. When it foams, add leeks, onions, celery and potato and sauté until the leeks and onions are translucent, 3 to 4 minutes. Add the chicken stock, reduce the heat to low, cover and simmer until the potato pieces are tender, about 15 minutes. Remove from the heat. Using an immersion blender, purée the soup in the pot. Alternatively, purée the soup in batches in a blender or food processor. Add the cream, place over medium-high heat and bring to just under a boil. Let cool, then strain through a fine-mesh sieve lined with cheesecloth. Transfer the soup to a clean pot and bring to a simmer over low heat. Cook, uncovered, to allow the flavors to blend, about 5 minutes longer. Do not allow to boil.
Ladle into warmed bowls, sprinkle with the pancetta and garnish with the breadcrumbs. Serve hot.