Over low heat, heat a 9-inch cast-iron skillet with a cover. Mix flour, baking powder, sugar, salt, cornmeal, and nonfat dry milk in a large bowl.
In a small bowl, whisk together egg, 3/4 cup water, and melted butter. Make a well in the center of dry ingredients, and pour the egg mixture into it. Stir until just combined.
Brush bottom, sides, and inside of skillet lid with vegetable oil; pour in the batter. Cover, and cook over low heat until top feels almost dry, about 35 minutes (check the bottom occasionally to make sure it's not burning; lower heat if necessary). Invert onto the skillet cover; oil pan bottom again. Slide bread back into pan, and cook 5 minutes more, until dry and light brown. Transfer to a plate, let cool slightly, and cut into wedges.