Melt the butter in a large saucepan. Add the leeks and cook over low heat, stirring occasionally, until softened, about 8 minutes. Add the potato and chicken stock and bring to a boil. Cover partially and simmer over low heat until the potatoes are tender, about 15 minutes. Add the cream and simmer for 10 minutes longer. Working in batches, puree the soup in a blender, then return it to the pan. Season the soup with salt and pepper, garnish with chervil and serve.