Melt the butter over medium heat, and add the minced pancetta to slowly render.
Add the vegetables to the pot, salt them lightly, and sweat them thoroughly for 15 to 20 minutes.
Destem the parsley, bruise the bay leaves and thyme, and lightly crush the juniper berries; bundle the stems, bay, thyme, and juniper with the reserved leek leaves into a bouquet garni.
Add the chestnuts, bouquet, and stock to the pot and bring to a boil; reduce to a simmer and let sit until chestnuts are softened, 25 to 30 minutes.
Remove bouquet and, working in batches or with an immersion blender, purée until smooth.
Stir in the cream and let thicken slightly over low heat.
Season to taste and serve garnished with chopped parsley.