In a large mixing bowl, combine warm milk, melted butter, sugar, and salt. Stir until sugar and salt are dissolved.
Sprinkle yeast over the mixture and let it sit for about 5 minutes, until foamy.
Add mashed potatoes and mix well.
Gradually add flour, one cup at a time, mixing until a soft dough forms.
Turn the dough onto a floured surface and knead for about 10 minutes, until smooth and elastic.
Place the dough in a greased bowl, cover, and let it rise in a warm place for about 1 hour, or until doubled in size.
Punch down the dough and divide it into 12 equal pieces. Shape each piece into a roll and place on a greased baking sheet.
Cover the rolls and let them rise for another 30 minutes, or until doubled in size.
Preheat the oven to 375°F (190°C). Bake the rolls for 20-25 minutes, or until golden brown.
Remove from oven and let cool on a wire rack.