Mix the flour, warm water, honey, and instant yeast until smooth and thick like batter. Scrape down the bowl, cover tightly, and set aside.
The Flour Mixture
Whisk together the flour, nonfat dry milk, and instant yeast. Sprinkle it evenly over the sponge without mixing, cover again, and let it ferment for 4 hours.
The Dough
After 4 hours, the sponge should be bubbling through parts of the flour mixture. Add the softened butter and mix on low speed with the dough hook until a rough, shaggy dough forms. Cover and let it rest for 20 minutes.
Sprinkle in the kosher salt and knead on medium speed for 7 to 10 minutes until the dough becomes smooth, shiny, and slightly tacky. It should pull away from the sides of the bowl but still stick lightly to your fingers.
If the dough feels too sticky, add a little flour. If it feels too stiff, add a small splash of water and knead again.
Lift the dough out, lightly grease the bowl, return the dough, and cover loosely. Let it rise for 1 to 2 hours, or until doubled in size.
Shaping and The Final Rise
Turn the dough onto a lightly floured surface and divide it evenly into two pieces.
Gently press each piece into a rectangle about the length of your loaf pan. It does not need to be exact.
Fold one side in slightly past center, then fold the other side over and pinch the seam closed.
Starting at the long edge closest to you, roll the dough into a loaf, gently pulling it toward you as you roll to create light tension. Pinch the seam closed.
If needed, gently roll the loaf from the center outward to match the length of the pan.
Place each loaf seam-side down in a lightly greased 9x5 loaf pan.
Cover and let rise in a warm spot for 1 to 2 hours, or until the dough rises about 1 inch above the pan.
Baking
During the final rise, preheat the oven to 350° and place a sheet pan on the lower rack so it heats with the oven.
Once the dough rises about 1 inch above the loaf pans, place the loaves in the oven above the hot sheet pan.
Carefully pour about ½ cup of ice onto the hot sheet pan, close the oven door, and bake for about 50 minutes, turning halfway through, until the loaves are golden and a skewer comes out clean.
Let the loaves cool on a rack for about 1 hour before slicing.