Whisk flour blend, yeast, sugar, and salt in a large bowl. Make a well and add warm milk, melted butter, egg, and vinegar. Stir until a sticky dough forms, then cover and let rise until puffy, about 30 minutes.

Dust a parchment-lined sheet and work surface with cornmeal. Pat dough into a 1-inch-thick rectangle, cut 8 rounds with a 3-inch cutter, and place them on the sheet. Dust tops with more cornmeal, cover, and rest 10 minutes.

Heat a cast-iron skillet over medium-low heat. Cook muffins in batches until deep golden, about 5 minutes per side, lowering heat if needed. Cool on a rack.

Split each muffin with a fork, pulling the halves apart to expose the craggy interior. Toast until golden and serve warm.