Use a spoon to scoop out the pulp from each potato half and transfer it, in batches, to a ricer or a food mill fitted with the finest blade. Force the potatoes through the ricer or mill into a microwave-safe bowl. Let cool, then cover and refrigerate overnight.
Just before serving, microwave the potatoes, covered loosely with plastic wrap, for 2 minutes. Stir and heat for another minute. Repeat the procedure until the potatoes are hot. Stir in 1 cup of the milk, the sour cream, the butter, and salt and pepper to taste, then heat for another minute, or until hot. Thin with additional milk if you prefer a lighter, softer texture. Serve immediately.
Nutrition information per serving: 230 calories; 50 calories from fat (22 percent of total calories); 4 g fat (3.5 g saturated, 0 g trans fats); 15 mg cholesterol; 40 g carbohydrate; 3 g fiber; 4 g sugar; 5 g protein; 230 mg sodium.