In the mixing bowl of a stand mixer, mix together the mashed potatoes, butter and beaten eggs. Add the yeast, sugar and milk. Mix on low speed just until combined. Let rest for 5 minutes or until bubbles begin to form on top.
With the mixer running, add the flour 1 cup at a time, until the dough comes together. The amount can vary depending on the stiffness of the mashed potatoes.
After all of the flour has been incorporated increase the speed of the mixer and knead until smooth and elastic around 5-7 minutes. Alternatively, turn the dough out onto a lightly floured surface and knead by hand for the same amount of time until smooth and elastic.
Form into a ball and place into a greased bowl. Turn to coat.
Cover and allow to rise in a warm draft free place until doubled in size, around 1 1/2-2 hours.
Punch down to release the air bubbles. Divide in half.
Place into 2 buttered 8 x 4 inch loaf pans. Cover and let rise again until doubled 1-2 hours.
Bake in a preheated 375°F oven for 25-30 minutes or until the bread sounds hollow when tapped in the middle. Check at 20 minutes and lay a piece of aluminum foil on top to prevent over browning, if needed.
Cool on a cooling rack.
Yeast equivalent - One [1/4 oz] packet is equal to [2 1/4 tsp] of dry yeast.