Combine 1 cup water, the cornmeal, molasses and 4 tablespoons butter in a saucepan. Bring to a simmer over medium-high heat, whisking constantly, until the mixture thickens and starts to bubble, about 3 minutes. Transfer to the bowl of a stand mixer and let cool until lukewarm (105 degrees F to 110 degrees F), stirring occasionally, about 20 minutes.
Sprinkle the yeast over 1/2 cup lukewarm water (105 degrees F to 110 degrees F) in a small bowl and let stand until foamy, about 5 minutes. Add to the cornmeal mixture along with 1 cup flour and the dry milk; mix with a wooden spoon. Cover with a kitchen towel and set aside in a warm place until the dough increases slightly in volume and is bubbly, about 30 minutes.
Mix the dough with the dough hook attachment on medium-low speed, then mix in the salt and the remaining 3 to 4 cups flour, 1/2 cup at a time, scraping down the bowl after each addition, until the dough comes together into a firm, tacky ball. Increase the speed to medium high; knead the dough until it pulls away from the bowl, about 2 minutes. Turn out onto a lightly floured surface and knead by hand until smooth and pliable, adding more flour as needed, about 5 minutes. Brush a large bowl with vegetable oil; add the dough, cover with a kitchen towel and set aside in a warm place until doubled in size, 1 hour to 1 hour, 30 minutes.
Brush a 9-by-5-inch loaf pan with vegetable oil. Punch down the dough and turn out onto a clean surface. Shape into a smooth 4-by-8-inch loaf, then transfer to the pan. Cover loosely with oiled plastic wrap and set aside in a warm place until the loaf rises above the pan by 1/2 inch, 30 minutes to 1 hour. Meanwhile, preheat the oven to 375 degrees F.
Uncover the pan and transfer to the oven. Reduce the temperature to 350 degrees F and bake until the bread is golden and sounds hollow when tapped, 35 to 45 minutes. (Cover loosely with foil if it is browning too quickly.) Brush with melted butter and let rest in the pan, 10 minutes. Turn out onto a rack and let cool before slicing or freezing.
MAKE IT AHEAD Let the bread cool completely, wrap in plastic wrap and foil, and freeze for up to 2 weeks. To serve, unwrap and thaw at room temperature for 2 hours. Reheat, wrapped in foil, at 300 degrees F until warmed through, 30 minutes.