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.