If not using leftovers, boil, drain (reserving water), and mash potato until smooth. Let cool. In a small bowl, combine the warm reserved potato water, 1 tsp sugar, and yeast. Let sit for 5-10 minutes until foamy.

In a large bowl, combine the mashed potato, melted butter, milk, remaining sugar, salt, and egg. Add the yeast mixture. Gradually add 4 cups of flour, mixing until a shaggy dough forms.

Knead the dough on a lightly floured surface for 8-10 minutes until smooth and elastic, adding more flour as needed to prevent sticking. Place in a lightly oiled bowl, cover, and let rise in a warm place for 1-2 hours, or until doubled.

Deflate the dough and shape it into a loaf. Place it in a greased 9x5 inch loaf pan. Cover and let it rise for another 30-60 minutes, until it's about 1 inch above the pan rim.

Preheat oven to 375°F (190°C). Bake for 30-35 minutes, until the crust is golden brown and the internal temperature is 190-200°F. Let cool in the pan for a few minutes before moving to a wire rack to cool completely before slicing.