"Why this recipe works:To get the flavor and texture of a doughnut with the ease of a muffin, we add an extra egg yolk to the batter and cut all-purpose flour with cornstarch to create a tender, cakelike crumb. A hot oven helps replicate the crispy edges of a traditional deep-fried doughnut, and a final coating of melted butter followed by a roll in cinnamon sugar creates a rich, sugary exterior...."