Make the sponge. In a mixer bowl or other large bowl, combine the flour, water, honey, and instant yeast. Whisk until very smooth, to incorporate air, about 2 minutes. The sponge will be the consistency of a thick batter, scrape down the sides of the bowl, and cover with plastic wrap.
Make the flour mixture and add to the sponge. In a medium bowl, whisk together the flour (reserve 1/4 cup if mixing by hand), dry milk, and instant yeast. Sprinkle this on top of the sponge and cover tightly with plastic wrap. Allow it to ferment for 1 to 4 hours at room temperature. (During this time, the sponge will bubble through the flour blanket in places: this is fine).
Mix the dough.
Add the salt and butter to the bowl and, with a wooden spoon or with your hand, stir until all the flour is moistened. Knead the dough in the bowl until it comes together, then scrape it onto a lightly floured counter. Knead the dough for 5 minutes, enough to develop the gluten structure a little, adding as little of the reserved flour as possible to keep the dough from sticking. Use a bench scraper to scrape the dough and gather it together as you knead it. At this point, it will be very sticky. Cover it with an inverted bowl and allow it to rest for 20 minutes. (This resting time will make the dough less sticky and easier to work with).
Knead the dough for another 5 minutes or until it is very smooth and elastic. It should still be tacky enough to cling slightly to your fingers a little. If the dough is still very sticky, however, add some of the remaining reserved flour, or a little extra.
Shape the dough and let it rise. Turn the dough out onto a lightly floured counter and cut it in half. Shape each piece into a loaf; the exact size is not important at this point. (A long side of the dough should be facing toward you). Dimple the dough with your fingers to deflate any large bubbles. Fold over the right side of the dough to a little past the center. Fold over hte left side of the dough to overlap it slightly. Press the center overlap section with the side of your hand to seal the dough. (If you have a lot of experiance shaping, you may prefer at this point to rotate the dough 90 degrees- a quarter turn). Starting at the top end of the dough, roll it over three or four times, until it reaches the bottom edge of the dough: with each roll, press with your thumbs to seal it and at the same time push it away from you slightly to tighten the outer skin. As you roll and press, the dough will become wider. If it is not as long as the pan, place both hands close together on top of the dough and, rolling back and fourth, gradually work your way towards the ends, gently stretching the dough. For the most even shape, it is important to keep a tight skin on the surface of the dough and not to tear it. If you want the edges of the loaf to be smooth, tuck the sides under. Place the loaves in the prepared loaf pans; the dough will be about 1/2 inch from the top of the pans. Cover them with a large container, or cover them loosely with oiled plastic wrap, and allow to rise until the center is about 1 inch above the sides of the pan, 1 1/2 to 2 hours. When the dough is pressed with a fingertip, the depression will very slowly fill inches.
Preheat the oven. Preheat the oven to 350°F 45 minutes before baking. Have an oven shelf at the lowest level and place a baking stone or a baking sheet on it, and a cast-iron skillet or sheet pan on the floor of the oven, before preheating.
Bake the bread. Quickly but gently set the pans on the hot baking stone or hot baking sheet. Toss 1/2 cup of ice cubes into the pan beneath and immediately shut the door. Bake for 50 minutes or until medium golden brown and a skewer inserted in the middle comes out clean (an instant-read thermometer inserted into the center will read about 210°F). Halfway through baking, turn the pans around for even baking.
Remove from oven and let cool on a wire rack.