Bring the milk, sugar, and salt to a boil in a small pot, stirring to dissolve the sugar. Add the butter and let it melt and then cool to room temperature. Dissolve the yeast in the warm water.
Put the sweet potatoes in your bread bowl and whip them vigorously with a fork, or use a standing mixer with a whisk attachment. Add the liquids (milk and dissolved yeast) and stir, then add 5 cups of flour and the cornmeal. Beat well, using your hands or a mixer with a dough hook, until you have a fairly stiff dough, then dump it out onto a floured surface and knead until smooth and glossy.
Put the dough in a well-oiled bowl, cover it, and leave it in a warm place until doubled, about 1 1/2 to 2 hours. Punch it down, pull it out onto a lightly floured surface, and divide it in half. Roll each piece out into a rectangle about 8 inches wide and 12 inches long. Start at the narrow side and roll the dough lightly and put it, seam-side down, in small (about 8 x 5-inch) bread pans. (You can also use larger loaf pans, of course: the bread won't fill the pans, but it'll still rise enough to be a loaf.) Cover with a damp cloth and leave the loaves in a warm place to rise until doubled, about 1 hour. Twenty minutes before baking, heat the oven to 400 degrees.
Bake for approximately 40 minutes, then remove from pans and cool on racks.