Melt the butter in a large (4-quart) saucepan over medium heat. Add the onions, season with salt and pepper, and cook gently, stirring occasionally, until translucent but not at all browned, 8 to 10 minutes. Add the garlic and cook another 2 minutes. Add the potatoes, stir, pour in the broth, and add the bay leaves. Bring to a simmer over medium-high heat, partially cover, reduce the heat to medium low or low and simmer gently until the potatoes are very tender, 25 to 30 minutes. Discard the bay leaves.
Working in batches, purée the soup in a blender (being careful to fill the blender only two-thirds full and vent the lid). If the soup is too thick to blend, add a little of the milk. Pulse the blender in short bursts and avoid overworking the soup.
Rinse the soup pot, and return the soup to it. Add the milk and cream and heat through. Season with salt and pepper to taste. Ladle the soup into bowls and top each with a generous handful of the pancetta croutons. Garnish with a thread of cream, if you like.