1 T (13 g) granulated sugar
1 cup (237 g) warm water (110°F-115°F) (using cold water will take longer to rise)
2 tsp (9 g) active dry or instant yeast
1/2 cup (113 g) sourdough starter, ripe (fed) or discard (ripe will give you a more vigorous rise)
3-1/2 cups (420 g) unbleached all-purpose flour 
1/4 cup (28 g) nonfat dry milk powder
2 T (29 g) unsalted butter, at room temperature
1-1/2 tsp salt
1/8 tsp citric acid optional; for enhanced sour flavor
Semolina flour or yellow cornmeal, for coating

Assemble all your ingredients (except semolina/cornmeal) in a large bowl.

Mix and knead — by hand, or with the dough hook on an electric mixer— to form a smooth dough. The dough should be soft and elastic; add additional flour (just a little) if it’s too sticky.

Place the dough in a lightly greased bowl, cover with plastic wrap, and set aside to rise for about 1-1/2 hours, or until it’s noticeably puffy. [For the most pronounced sour flavor, cover the bowl and immediately place it in the refrigerator (without rising first). Let the dough chill for 24 hours; this will develop its flavor.]

Gently deflate the dough, then turn it out onto a lightly floured work surface, cover it, and let it sit for a few minutes, to relax the gluten. Gently roll it out approximately 1/2″ thick and cut 3″ rounds (the rim of a 3” wide glass works well in a pinch).  Gather any remaining scraps, reroll and cut until the dough is used completely.

Sprinkle some semolina over a baking sheet and place the dough rounds on it, equally spaced (don’t let them touch). Sprinkle them with additional cornmeal or semolina, cover with plastic wrap, and let them rise until light and puffy, 45 to 60 minutes. If the dough has been refrigerated overnight, the rise time will be about 2 hours.

Carefully transfer the rounds (without crowding them) onto a large electric griddle preheated to 350°F, or to an ungreased frying pan that has been preheated over medium-low heat.

Cook the muffins for 5-7 minutes on the first side then lay another sheet pan, or similar flat pan (though not overly heavy) atop them. Continue cooking for 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).

An instant-read thermometer inserted in the center of a muffin should register 190°F. The edges may feel a bit soft; that’s OK. If the center isn’t quite done, you can also transfer them to a preheated 350°F oven and bake until the muffins’ interior shows no sign of wet dough, an additional 5 minutes or so.

Remove the muffins from the griddle, and cool on a rack. Store at room temperature, tightly wrapped, for 4 or 5 days. These keep wonderfully in the freezer too!

Adapted from a recipe over at King Arthur Flour.