In a large pot, melt 3 tablespoons of the butter over low heat. Add the leeks and cook, stirring often, until tender, about 3 minutes. Add the garlic and cook for 30 seconds. Add the chicken stock and potatoes. Cover and simmer gently until the vegetables are tender, about 30 minutes.
Remove from the heat. With a hand-held immersion blender, or in batches in a food processor, puree the soup until smooth. Season, to taste, with salt and ground white pepper. Ladle into soup bowls and garnish with parsley and 1 tablespoon of soft herbed cheese. Serve immediately.