• Bake the sweet potato until tender. I bake mine super-quick in the microwave for 6 - 8 minutes in the microwave oven. 

  Scoop the potato from the skin and thoroughly mash or put it through a potato ricer.

Sponge

• 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 wrap and set aside.

Sweet Potato Bread Dough

• In a separate smaller bowl, thoroughly whisk together the flour, milk powder, and yeast. Using a large spoon, sprinkle this mixture over the top of the Sponge without mixing it in, and cover again with plastic wrap.

  Set the bowl in a warm area of the kitchen to ferment for a couple of hours. The sponge will bubble up through and around the dry mixture covering it.

  Pro tip: I heat a cup of water in my microwave oven and place it in the back corner. Then I set the bowl containing the dough in the microwave and close the door.
• Scrape the Sponge and all of the dry mixture into the bowl of a standing mixer, fitted with a dough hook attachment. Add the butter and ½ cup of the mashed potato (it will likely be the entire potato).

  Mix the dough at a low speed until the flour is moistened, and forms a rough ball. Cover with plastic wrap and let it sit for 20 minutes at room temperature.
• Add the salt and increase the speed of the mixer to a medium speed. Continue to knead the dough using the dough hook attachment until the dough is smooth and shiny.
• First rise. Oil a large clean bowl, scrape the dough into the bowl, and cover with plastic wrap. Set in a warm area of the kitchen free of drafts to allow the dough to rise and double in size (about 1 ½ - 2 hours).
• Fold. Gently scrape the dough onto a lightly floured 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 of dough.
• Second rise. Return the dough to the oiled bowl, cover it with plastic wrap, and return it to a warm area of the kitchen free of drafts. Let the dough double again in size (about 1 ½ - 2 hours).
• Preheat the oven. Preheat the oven to 400˚F 1 hour before baking the bread, and set your oven rack on the bottom rung. Place a baking stone on the rack if you have one. Otherwise, place a baking sheet on the rack. If you use a baking sheet, you don't need to preheat the oven 1 hour before baking the bread.
• Final fold and rise. Turn the dough onto a lightly floured space, slightly flatten it into a rectangle, and fold it again into thirds like a business envelope. Tuck under the ends to form a loaf.

  Butter a loaf dish, and place the dough in the dish. It won't fill it yet. Cover loosely with plastic wrap, and allow the dough to rise one final time. It may dome above the sides of the loaf pan, so be careful the dough doesn't over-rise.
• Bake. Place the loaf pan onto the baking stone or baking sheet 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 loaf reads 190-200˚F.
• Unmold and cool on a wire rack.