Cereal Mash (Traditional American "Double Mash")
American brewers more than a hundred years ago realized that domestic barley had an excess of protein and that corn and rice, with their low protein levels, could be used to their advantage. However, corn and rice starches don't gelatinize at mash temperatures, and aren't available to the malt enzymes for conversion into sugars. Boiling the cereal gelatinizes the starch, but then you have cooked rice or cornmeal mush, and those are hard to handle in a brewery. Plus, when they cool, they become really stiff and hard to move or incorporate into the mash. The secret turned out to be malt. By adding a small amount of malt to the cereal and mashing a short time before cooking, the cereals become quite thin and stay that way.
The practice one hundred years ago was to use 30% malt by weight despite current practice of using only 10%. In a kitchen pot with lid, mash about five ounces of malt for every pound of corn meal, grits, polenta, or coarsely ground rice. Use about a quart and a half of treated mash water per pound of corn, two quarts for rice. Rest at about 153°F (67C) for 20 minutes in a preheated oven or wrap well in a blanket, then bring to a boil on the stove or your brewery burner.
Rice and corn meal should be cooked covered about 30 minutes; grits or polenta 45 minutes to an hour. Stir as you bring them up to a boil and occasionally during the boil, adding more water if necessary. It's best not to overcook rice, but corn can be cooked longer for more flavor and color reactions to take place in the cooker if you want these.
Meanwhile, you have started the main, or malt mash, and timed it so just as the cereal mash is done, it is time to boost the temperature of the main mash. It's best to plan this ahead on paper.
The recommended mash schedule for the main mash is: mash in at 145°F, rest for 30-45 minutes while boiling the cereal mash, add the cereal mash, which brings the combined mash to about 160°F (you may need to add some boiling water or heat), and rest another 30-45 minutes. This produces a wort with high fermentability for a crisp pilsner.
Be sure to use de-germed cornmeal, not whole corn meal, which has the oily germ present.
This information was gleaned from a Zymurgy article and subsequent HomeBrew Digest posts by Jeff Renner.