Feed the sourdough starter at a 1:1:1 ratio and let it ferment until doubled and visibly bubbly, about 6 to 8 hours.

Check that the starter is active by confirming that it floats in water before measuring 100 g for the dough.

Combine the active sourdough starter, sugar, milk, all-purpose flour, and fine sea salt in a large bowl until no dry flour remains.

Cover the dough and rest it for 30 minutes at room temperature.

Stretch and fold the dough once, then cover it and rest for 30 minutes.

Perform a coil fold or a second stretch and fold, then cover the dough and rest it for 30 minutes.

Stretch and fold the dough a third time, then cover it again.

Refrigerate the dough during bulk fermentation if needed to slow its rise, then return it to room temperature to finish doubling.

Turn the risen dough onto a lightly floured work surface and dust its top with a small amount of all-purpose flour.

Press the dough with your fingertips until it is approximately 1 inch thick.

Sprinkle parchment paper squares with about half of the cornmeal and place them on a work surface.

Cut the dough into 3-inch rounds and transfer each round onto the cornmeal-coated parchment squares.

Lightly mist the tops of the rounds with water and sprinkle them with the remaining cornmeal.

Cover the shaped muffins with a kitchen towel and proof them at room temperature for 60 minutes.

Preheat a non-stick skillet over low heat while the muffins finish proofing.

Place the muffins in the dry skillet in batches and cover the skillet.

Cook the muffins for 4 minutes on the first side over low heat, keeping the skillet covered.

Flip the muffins and cook for 4 minutes on the second side until golden and cooked through.