1 1/4-ounce envelope active dry yeast (about 2¼ teaspoons)
1 tablespoon sugar
1 cup buttermilk, warmed
4 tablespoons (1/2 stick) unsalted butter, room temperature
2 tablespoons vegetable oil
2 teaspoons kosher salt
3 1/2 cups unbleached bread flour, divided, plus more
Nonstick vegetable oil spray
Cornmeal (for dusting)
Whisk yeast, sugar, and 1 cup warm water in the bowl of a stand mixer fitted with the dough hook. Let sit until foamy, about 5 minutes. Add buttermilk, butter, oil, salt, and 3½ cups flour and beat on low speed until a shaggy dough forms. Increase speed to medium and beat until dough is smooth and begins to pull away from the sides of the bowl, but is very wet and sticky, about 5 minutes.
Spray a medium bowl with nonstick spray, transfer dough to greased bowl, and cover tightly with plastic wrap. Chill overnight (the cold rise makes the dough easier to handle, produces better flavor, and creates more nooks and crannies).
Dust 2 parchment-lined rimmed baking sheets generously with cornmeal (you want to cover all the parchment). Spray a clean work surface with nonstick spray, turn out dough, then spray dough. Using a bench scraper, divide dough into 12 equal pieces (they should be about 3 oz. each). Working with 1 piece at a time and using bench scraper, fold dough inward onto itself on 4 sides. Turn dough over with bench scraper so that folds are underneath and transfer to prepared sheet. Repeat with remaining dough, spacing evenly on sheets.
Sprinkle each piece with cornmeal. Spray plastic wrap with nonstick spray and loosely cover baking sheets, oiled side down. Let sit at room temperature until dough is nearly doubled in size, 60–70 minutes.
Heat a large skillet or griddle, preferably cast iron, over low. Place another rimmed baking sheet in the center of oven and preheat to 350°. Working in 2–3 batches and keeping remaining dough covered, slide 2 thin metal spatulas from opposite sides underneath dough, also getting underneath cornmeal to avoid sticking or deflating dough, and transfer to skillet (do not overcrowd). Cook until bottoms are dark golden brown, 5–7 minutes. Turn and cook until other side is dark golden brown, 5–7 minutes. Using spatula, transfer muffins to preheated sheet in oven and bake until cooked through and sides are dry to the touch but still spring back, 5–10 minutes. Transfer to a wire rack. Repeat with remaining dough. Let muffins cool at least 30 minutes before splitting with a fork and serving.
Do Ahead: English muffins can be made 1 day ahead. Let cool and store in a resealable plastic bag at room temperature. Toast lightly before serving.