1cupsourdough starter discard, unfed, at room temperature, see Recipe Notes
2cupsmilk, warmed between 100°F to 110°F, see Recipe Notes
¼cupunsalted butter, at room temperature
1tablespoonyeast, instant or rapid rise, see Recipe Notes
cornmeal or semolina, for coating
Make the dough: Combine all the dough ingredients (except for the cornmeal or semolina) in a stand mixer bowl. Using the dough hook, knead to form a smooth dough. You can also knead the dough in a bread machine or by hand. The dough should be soft and elastic, but not particularly sticky, so add additional flour if needed (use 1 tablespoon increments until you get the desired consistency. This is especially necessary if you’re baking on a humid day).
Place the dough in a lightly greased bowl, cover, and immediately place it in the refrigerator (this is called retarding the dough. This allows the sourdough to ferment slowly and develop a more pronounced sour flavor. Let the dough chill for 8 hours (or up to 2 days). If you don’t want to cook the muffins on a separate day, just set the covered bowl aside to rise for about 1½ hours, or until it's noticeably puffy.
Shape the muffins: Gently deflate the dough, turn it out onto a lightly floured work surface, cover it, and let it sit for a few minutes to relax the gluten. If you’ve retarded the dough in the refrigerator, allow the dough to rest for about an hour on the counter to warm to room temperature.
Option 1: Divide the dough in half. Working with one piece at a time, roll ½-inch thick, and cut in rounds using a 3-inch round cookie cutter. Re-roll and cut any remaining scraps. Repeat with the remaining half of dough.
Option 2: Divide the dough into 24 equal pieces (I weighed the dough using a kitchen scale and divided by 24, then cut pieces to that weight). Shape each piece into a round ball, then flatten each ball into a 3-inch round. For a somewhat more even rise as the muffins cook, flatten each ball slightly larger than 3 inches, and trim edges with a 3-inch round cookie cutter (or trim all around the edge with a pair of scissors). Muffins with cut (rather than flattened) sides will rise more evenly, and I used the trimmings to get extra pieces. Win!
Place the rounds, evenly spaced, onto a cornmeal- or semolina-sprinkled half sheet baking pan (12 per sheet). Sprinkle them with additional cornmeal or semolina, cover with plastic wrap, and let them rise on the counter until light and puffy, about 45 to 60 minutes. If the dough has been refrigerated overnight and you didn’t let it come to room temperature first, the rise time will be about 2 hours.
Cook the muffins: Carefully transfer the rounds (as many as a time that will fit without crowding) to a large electric griddle preheated to 350°F, or to an ungreased griddle or frying pan that has been preheated over medium-low heat.
Cook the muffins for about 10 to 12 minutes on each side. For the best shape, cook muffins for about 5 minutes on their first side; then lay a quarter sheet baking pan, or similar flat (though not overly heavy) object atop them. Continue cooking for 5 to 7 minutes or so; then remove the pan, turn muffins over, and finish cooking without the pan on top. This helps keep muffins flat across the top (rather than domed). They’re done when a digital thermometer inserted in the center of a muffin registers 190°F. The edges may feel a bit soft; that's OK.
Remove the muffins from the griddle, and cool on a rack. Store tightly wrapped at room temperature for 4 or 5 days; freeze for longer storage.
Using fed sourdough starter can give you a better rise (depending on how vigorous your starter is), but using unfed sourdough starter discard works just fine. If you do use fed starter (and it's active and bubbly), you can omit the yeast, but the rising times can be longer.If you use Active Dry yeast, then you might want to sprinkle yeast on to the warmed water with a teaspoon of sugar and allow it to sit for a few minutes before proceeding with the recipe. Letting it foam "proves" the yeast is active and ready to go to work.This recipe is easily halved using 2 teaspoons yeast and ½ cup unfed sourdough starter.I used Trader Joes 1% milk that I warmed (brought to just a simmer). The original recipe called for 2 cups warm water with ½ cup (43g) nonfat dry milk, so that’s an option as well. the milk should be just warm to the touch. Too hot, and you kill the yeast.The original recipe noted that you can also use your ripe (fed) sourdough starter, and that it would give you a more vigorous rise. If I have ripe starter, I’m making sourdough bread...my sourdough starter discard rises quite well, thank you very much, so I prefer using the discard.If you're having trouble getting the muffins to cook all the way through on the stove top, cook until golden brown on both sides, then transfer to a preheated 350°F oven and bake until the muffins' interior shows no sign of wet dough, about 10 minutes or so.