Cook potatoes: Fill a medium size saucepan with 2 to 3 cups of water and add the potato cubes. Bring to a boil over medium-high heat and cook for 10 to 15 minutes or until fork tender. Drain and mash it with a fork.
Heat the milk and add the potato: Add the milk into a medium size saucepan and add the butter and salt to it. Heat on medium heat just until the milk start to bubble around the edges a bit. Stir it then add the mashed potatoes. Pour this mixture into the bowl of your stand mixer and let it cool until still warm but not hot to touch.
Make the dough: Add the yeast to the mixer bowl and 2 cups of flour. Using the paddle attachment mix until the flour is fully incorporated into the milk mixture. Switch to the dough hook and add another 2 cups of flour. Mix on low speed for 5 to 7 minutes. Add remaining flour as needed, it should mostly come clean from the sides of the bowl but should still be loose and a little gluey.
First rise: Grease a large bowl with cooking spray then place the dough in the bowl. Cover with plastic wrap or a damp clean towel and let it rise in a warm draft-free spot until doubled in size. You can also refrigerate the dough overnight, it should slowly rise in the fridge.
Place dough in loaf pans: Punch down the dough with your fist to let the air out. Turn the dough onto a clean and lightly floured surface. Knead it with your hands a couple time, then using a pastry scraper, cut it into two equal size pieces. Great 2 8x4-inch loaf pans with a little oil or cooking spray. Place each half into the greased pans.
Second rise: Lightly sprinkle the top of the loaves with a bit of flour and if preferred, cut 3 slits into each loaf. Let the loaves rest in a warm draft-free environment for another 30 minutes to an hour or until doubled in size.
Bake: While the loaves are resting, preheat the oven to 375°F. Transfer the loaf pans to the oven and bake for 35 to 40 minutes or until the bread sounds hollow when lightly tapped. If the loaves are browned on top but still not baked through, cover them loosely with foil to prevent over-browning and continue baking until done. The breads are baked through when the internal temperature is between 190°F and 200°F. Let the loaves cool for about 2 minutes in the pans, then gently run a dull knife along the edge of the pan to loosen the bread. Turn the loaf pans onto a cooling rack and let cool completely before slicing and serving.