1. Melt the butter in a large saucepan. Add the leeks, onions and two teaspoons of salt and cook over moderate heat, stirring occasionally, until the vegetables are softened, about 10 minutes.
2. With an immersion blender puree the soup (you can use a blender too) until very smooth. Add the sour cream and bring to just a simmer over moderate heat, stirring occasionally. Season with salt. Ladle the soup into bowls. Garnish with the chives and pepper and serve.
3. Stir in the brandy and wine and boil until nearly evaporated, about five minutes. Add the potatoes and water and bring to a boil. Simmer over moderate heat until the potatoes are very tender, about 35 minutes.