The recipe calls for a huge amount of levain, some yeast, some milk, a little oil, and a touch of sugar. The dough is almost batter-like, and while English muffins are typically cooked in rings on a griddle, we wanted to give these time to rise, so we use a mold where they proof for about half an hour before going into a hot oven. Of course, cornmeal goes into the molds first, for the classic English muffin bottom.
*Note: You'll need a 13-by-21-inch rimmed baking sheet, 9-10 pounds of golf ball-sized nonsedimentary river rocks (which can be purchased from stone yards, landscape or garden supply stores, and home-improvement centers), 10 feet of metal chain link, and a water gun that can deliver about 1 ½ cups water quickly and from far enough away that the steam doesn't burn you (preferably a Super Soaker).
Place all of the stones and the chain in the pan and set the pan on the bottom of the oven (as long as there is no heating element there) or on the lowest rack. Position a rack above the pan, making sure there's enough room that you'll be able to hit the rocks and chain with the water, and put the baking stone on this rack. Remove all of the other racks.
Preheat the oven for at least 1 hour, preferably 2 hours before baking. Meanwhile, proceed with the remaining steps for the muffins.
Combine 250 grams of the flour and 250 grams of the water in a plastic or glass container large enough to contain the ingredients comfortably and stir with your hand until uniformly combined. Cover the container. If it has a lid, leave it loose to let out the gas the yeast will produce; if you're using plastic wrap, leave it loose or poke some holes in it.
Put the container in a place that's most likely to maintain a steady 72-78 degrees (i.e.: not in a cold basement or a sunny window); the top of the refrigerator is often an excellent place to keep your levain. (After 24 hours, you will start to see bubbles — not a lot, but enough to let you know some yeast is active in there.)
The next day, combine 250 grams of the flour and 250 grams of the water in a plastic or glass container. Add 150 grams of the 24-hour-old starter and discard the rest. Stir to combine well, then cover and return to the same place.
Repeat the feeding every 12 hours, using 250 grams of the flour, 250 grams of the water, and 150 grams of the starter (discard the excess starter). (You should notice an increasing amount of bubbles at each feeding, indicating increased yeast activity.) After 6 feedings, the starter should be sufficiently developed to use, but it will grow stronger with continued feedings and will be at optimal strength after 2 weeks of twice-daily feedings.
Set up a baking stone and kit and preheat the oven to 425 degrees.*
Combine the flour, sugar, yeast, and salt in the bowl of an electric stand mixer fitted with the paddle attachment and give it a quick mix on the lowest setting to distribute the yeast evenly.
Make a well in the center, add the levain, and mix on low speed to begin to moisten the dry ingredients, for about 2 minutes. Slowly add the milk, followed by the oil, and mix until smooth, for about 2 minutes.
Scrape down the sides and bottom of the bowl and pulse to incorporate. Increase the speed to medium and mix for 2 minutes. Let the dough rest at room temperature, uncovered, for 1 hour.
Spray a Flexipan cylinder mold with cooking spray and place on a baking sheet. Sprinkle ½ teaspoon of the cornmeal in each cup.
Scoop about ¼ cup of the dough into each cup. Cover with a plastic tub or a cardboard box and proof until the dough has risen ¼ inch over the tops of the molds, for 30-45 minutes. When the top of a muffin is touched, it will begin to collapse on itself.
Wet your finger and gently smooth any uneven spots on the surface of the muffins. Place the pan on the baking stone. Immediately spray water onto the steam generator (rocks and chain). Quickly shut the oven door and bake until the muffins are a rich golden brown, for 20-25 minutes. Then, let cool completely on a cooling rack.