For the preferment, whisk 190g of all-purpose flour, ¾ teaspoon of instant yeast, 152g of lukewarm whole milk, and 102g of lukewarm water, both about 90°F / 32°C, until smooth. Cover and leave at room temperature for 1-4 hours.
Add 120g of all-purpose flour, 1 teaspoon of table salt, 1 teaspoon of sugar, and 18g of melted unsalted butter, and mix into a soft, sticky dough.
Stretch up one edge of the dough and fold it over, working around the bowl, 6-8 times, until it resists stretching. Cover and rest at warm room temperature for 1 hour.
Repeat the folds, cover, and rest for about 1 hour more, until very puffy.
Dust a work surface and a sheet of parchment paper with 20g of yellow cornmeal. Turn the dough onto the surface, pat it ½in / 1cm thick, and cut out rounds with a 3½in / 9cm cutter, pressing the scraps together to cut more.
Set the rounds on the parchment, cover, and rest for 20-30 minutes, until puffy.
Heat a 10-12in / 25-30cm cast-iron or nonstick skillet over medium-low heat until a few drops of water sizzle. Cook the muffins for 10-15 minutes per side, until golden brown.
