Preparation Melt butter in heavy large saucepan over medium heat. Add leeks; sauté until just beginning to color, about 15 minutes. Add potatoes and water. Season with salt. Increase heat to high; bring to boil. Cover; boil until potatoes are tender, about 20 minutes. Uncover; boil until almost all liquid evaporates, about 5 minutes. Mash potatoes coarsely; mix in sour cream. Season with salt and pepper. (Can be made 2 hours ahead. Cover; let stand at room temperature. Rewarm over low heat, stirring.) Transfer to large bowl.
Preparation Melt butter in heavy large saucepan over medium heat. Add leeks; sauté until just beginning to color, about 15 minutes. Add potatoes and water. Season with salt. Increase heat to high; bring to boil. Cover; boil until potatoes are tender, about 20 minutes. Uncover; boil until almost all liquid evaporates, about 5 minutes. Mash potatoes coarsely; mix in sour cream. Season with salt and pepper. (Can be made 2 hours ahead. Cover; let stand at room temperature. Rewarm over low heat, stirring.) Transfer to large bowl.