Spray a 9-by-5-inch loaf pan with nonstick spray and sprinkle the cornmeal all over the inside of the pan. Tap out any excess.
Add the flour, sugar, salt, baking soda and yeast to a large bowl and whisk to combine. Set the dry mixture aside.
Add the milk and butter to a small saucepan and attach a candy thermometer. Turn the heat on low and bring the mixture up to 125 degrees F. If the mixture gets too hot, let it cool to the correct temperature before continuing.
Pour the warm liquid over the dry ingredients and use a rubber spatula to mix until the dough comes together, but is still shaggy. Turn the dough out onto a lightly floured work surface and knead until the dough is soft and elastic, 2 to 3 minutes. Form the dough into a log and transfer it to the prepared loaf pan, seam-side down. Cover the dough with a clean kitchen towel and let it rise in a warm area until it's puffed up and almost doubled in size, about 1 hour.
While the dough is rising, preheat the oven to 400 degrees F.
Bake the loaf until the top of the bread is golden and the internal temperature is 190 degrees F, 25 to 30 minutes. Allow the bread to cool in the pan for 10 minutes, then remove the bread from the pan and cool completely on a wire rack before slicing.