Combine the flour, dry buttermilk powder, warm water, salt, and yeast. Mix and stir everything together to make a very sticky rough dough.
Cover the bowl and let the dough rise at room temperature for 2 hours. Then refrigerate it for at least 2 hours, or for up to about 7 days. The longer you keep it in the refrigerator, the tangier it gets. If you chill it for 7 days, it will taste like sourdough.
When you're ready to make the bread, sprinkle the top of the dough with flour to make it easier to grab. Grease your hands and split the dough in half. Take half of the dough at a time, and plop it on a floured work surface, and round it into a ball, or into a long log.
Place the dough on a piece of parchment (if you are going to use a baking stone), or onto a lightly greased or parchment-lined baking sheet, or on a bread pan. Sift a light coating of flour over the top to help keep the bread moist as it rests before baking.
Let the loaves warm to room temperature and rise. This should take about 60 minutes. It won't appear to rise upwards, rather it will seem to settle and expand.
Preheat your oven to 450ºF while the loaves rest. Place a shallow metal or cast iron pan (not glass, Pyrex, nor ceramic) on the lowest oven rack, and have 1 cup of hot water ready to go.
When you're ready to bak, take a sharp knife and slash the bread 2 or 3 times, making cuts about ½ inch deep.
Place the bread in the oven - onto the baking stone, or in the middle of the middle rack if it's on a pan. Carefully pour the 1 cup of hot water into the shallow pan on the rack beneath the bread. It'll bubble and steam; close the oven door quickly.
Bake the bread for 25 to 35 minutes, until it's a deep, golden brown.
Remove the bread from the oven and cool on a rack. Store leftover bread in a plastic bag at room temperature. (or it can be frozen).