In 6-quart or larger saucepan, heat 4 quarts water to boiling over medium heat. Meanwhile, in 2-quart microwavable bowl, mix potatoes, milk and melted butter. Cover; microwave on High 4 to 6 minutes or until steaming. Uncover; let stand 2 minutes. Stir in egg. Stir in 1 cup of the flour and the baking powder.