For the bread dough: Place the warm water in a large bowl and sprinkle in the yeast. Let sit until foamy, about 5 minutes. Add the egg, egg yolks, oil, sugar, 4 cups of the flour, and the salt. Stir with a wooden spoon until the mixture comes together to form a shaggy dough and most of the flour is incorporated. Turn out onto a lightly floured work surface and knead, adding more flour a little at a time if the dough is sticky or very soft (it should be firm). Knead until the dough is smooth and elastic (when you push down, the dough should feel firm and push back), about 10 to 15 minutes. Coat a large bowl with a thin layer of vegetable oil. Place the dough in the bowl, turn to coat in the oil, and cover with a damp tea towel or plastic wrap. Let rise in a warm, draft-free area until tripled in volume, about 40 to 60 minutes. When fully risen, an indentation made with a finger pushed into the center of the dough should remain; this is a fully aged, or ready, dough. Punch down the dough, cover, and allow to rise for 15 minutes more. Punch down again and cut into 2 equal portions. Cover 1 portion with a damp tea towel or plastic wrap; set aside. Divide the other portion into 6 equal pieces. On a work surface, use your palms to roll the pieces into 6 ropes that are thick in the center, tapered to a point on each end, and at least 12 inches long (lightly flour the work surface if the dough becomes too sticky or difficult to roll). Line up the 6 strands vertically and pinch the top ends together. Counting from left to right, bring strand number six from the right end over strand number one on the left and up to the left. Bring strand number one from the left up to the top right. You now have a four-legged creature with two arms crossed over each other. Keeping the legs spread apart in pairs, the left arm (as you face it) comes down into the center between the legs. Bring the outer right leg over and up to form a new top left arm. The top right arm comes down between the legs. Repeat the pattern: The left outer leg comes up to form the new right arm, and the left arm comes down to the center. The right leg comes up to form the new left arm, and the right arm comes down to the center. Continue alternating (left leg up to become right arm, left arm down to the center; right leg up to become left arm, right arm down to the center, and so on), always keeping hold of the last strand you moved so that you remember your location in the pattern. When moving strands, grasp the arms by the ends where hands would be. Grasp the legs where feet would be. Keep the legs spread in pairs so that the arms can easily be brought down to the center. If you become momentarily disoriented when braiding, remember that the arms come down, and the legs come up. If you make an error and become entirely confused, stop, open the braid, and begin again. When done, if there is any doubt, turn the bread upside down. The design should be perfectly symmetrical. If not, undo it and begin again. Finish by pinching the ends tightly closed. Dust a baking sheet with the cornmeal. Transfer the challah to the baking sheet and repeat the process with the second portion of dough. Place the challahs in a warm, draft-free area and allow to rise until almost doubled in size, about 45 minutes. For the egg wash and topping: Heat the oven to 350°F and arrange a rack in the middle. Combine the beaten egg, water, and salt. Brush both loaves with the egg wash, using care to cover them completely, but do not let any excess egg drip into the crevices. Let the loaves sit until dry, about 15 minutes, and then brush again. Sprinkle with poppy seeds or sesame seeds if desired. Bake the loaves, rotating the sheet halfway through the baking time, until they are a rich mahogany color and emit a hollow sound when tapped lightly on the bottom with your fingertips, about 35 minutes. If the tops begin to brown excessively and the bottoms are raw, cover the bread with a sheet of parchment paper or aluminum foil that has been creased down the center to form a tent. If there is a white line visible between the braids, continue baking until it disappears. To test for doneness, press lightly between the braids on the highest part of the bread; it should be firm. If you feel the creases give when lightly pressed, continue baking until they firm up. Let cool on a wire rack. Challah keeps very well for several days in a plastic bag in a breadbox. It can also be wrapped in plastic or foil and placed in a resealable freezer bag, frozen, and defrosted slowly overnight in the refrigerator.
