Combine all of the Sponge ingredients in a large bowl, and whisk thoroughly until smooth. It will be the consistency of a thick batter. Cover with plastic while you make the Flour Mixture.
Bake the sweet potato until tender. I bake mine super-quick in about 6 - 8 minutes in the microwave oven.
In a separate smaller bowl, combine the following Flour Mixture ingredients - flour, milk powder, and yeast - and thoroughly whisk so that the yeast is evenly dispersed in the dry ingredients. Using a large spoon, sprinkle this dry mixture over the top of the Sponge, and cover with plastic. Do not whisk or otherwise combine at this point. Set the bowl in a warm area of the kitchen to ferment for a couple of hours. I always heat a cup of water in my microwave oven, slide it to a back corner, and place the bowl in the microwave. The sponge will bubble up through and around the dry mixture covering it.
Scrape the sponge and dry mixture into the bowl of a standing mixer, fitted with a dough hook. Add the butter and 1/2 cup of the mashed potato (it will likely be the entire potato). Mix into a dough at low speed until the flour is moistened, and forms a rough ball. Cover with plastic and let it sit for 20 minutes.
Add the salt and knead with the dough hook at a medium speed until the the dough is smooth and shiny.
Oil a bowl, scrape the dough into the bowl, and cover with plastic. Set in a warm area of the kitchen free of drafts to allow the dough to rise and double in size (about 1 1/2 - 2 hours).
Gently scrape it onto a lightly floured work surface, and smooth the dough out into a rectangle. Fold the dough as if you are folding a business letter. Turn the dough 90˚ and fold again like a business envelope. You'll end up with a thick packet. These folds are important for developing structure in the finished bread. Return it to the oiled bowl, cover with plastic, and return it to a warm area of the kitchen free of drafts. Let the dough double again in size, about 1 1/2 - 2 hours.
Preheat the oven to 400˚F 1 hour before baking the bread, and set the rack you'll use at the bottom of the oven, or one rung up from the bottom. Place a baking stone on the rack.
Turn the dough onto a lightly floured work space, slightly flatten into a rectangle, and fold into a business envelop one final time. Tuck under the ends so that a loaf is formed. Butter a loaf dish, and place the dough in the dish. It won't fill it yet. Cover with plastic, and allow to rise one final time. It should dome above the sides of the loaf pan.
Slide the loaf pan onto the baking stone and bake for 5 minutes. Lower the temperature to 375˚F and continue to bake for 15 - 20 minutes, or until the temperature probe inserted into the middle of the bread reads 190-200˚F.
Unmold and cool on a wire rack before eating.