In a measuring cup, combine water, sugar and yeast, let sit for 10 minutes. In a large bowl, combine flour and salt. Add in yeast mixture, warm milk, egg, melted butter and mashed potatoes. Mix until a dough forms.
Knead dough on a floured surface until smooth. Place dough into a large greased bowl, turning to coat. Cover and let rise for 1-2 hours, or until doubled in size.
Preheat oven to 400 F. Deflate dough and divide into even pieces. Roll each piece into a ball and place into a lightly greased baking dish. Cover and let rise for another 30-60 minutes. Bake for 15-18 minutes.