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