1. Bring potatoes, 1 teaspoon of the salt, and enough cold water to cover by 2 inches to boil in large saucepan. Reduce heat and simmer 18 to 20 minutes, until potatoes are fork-tender.
2. Meanwhile, melt 4 tablespoons of the butter in small skillet over medium heat. Add shallots; cook until shallots are lightly golden and softened, about 5 minutes. Set aside.
3. Drain potatoes. Return to saucepan and cook 2 minutes over low heat to dry. Coarsely mash potatoes with potato masher or large wooden spoon. Stir in shallot mixture, milk, sour cream, remaining 1/2 teaspoon salt and pepper, mashing until well combined and heated through. Cut up remaining 1 tablespoon butter and swirl into potatoes. Transfer to serving bowl. Makes 5 cups (5 servings).