Heat butter in a large stock pot or Dutch oven over medium heat.
Add leeks and garlic and cook 3 minutes, until soft. Add thyme and marjoram and cook 1 minute, until fragrant. Add potatoes and broth, increase heat to high and bring to a boil. Reduce heat, partially cover and simmer 5 minutes.
Using an immersion blender, puree until smooth (or puree in a blender and return mixture to pot). Remove from heat and whisk in sour cream. Season, to taste, with salt and black pepper. Ladle soup into bowls and top with chives.
Copyright 2006, Robin Miller, All Rights Reserved