Reduce the cider. Pour the apple cider into a small saucepan and simmer over medium heat until reduced to about 1/2 cup, 15–20 minutes. Cool completely. This step intensifies the apple flavor.
Mix dry ingredients. In a large bowl, whisk flour, cornstarch, baking powder, baking soda, cinnamon, nutmeg, and salt until evenly combined.
Combine wet ingredients. In another bowl, whisk the melted butter, granulated sugar, and brown sugar until sandy and cohesive. Add eggs, one at a time, then whisk in the cooled reduced cider, buttermilk, and vanilla.
Bring the dough together. Pour the wet mixture into the dry ingredients. Stir with a spatula until no dry patches remain. The dough will be soft and slightly sticky. Cover and chill 45–60 minutes to firm up for easier handling.
Prep the coating. Stir together sugar, cinnamon, and salt in a shallow dish. Keep the melted butter in a separate small bowl.
Heat the oil. In a heavy pot or Dutch oven, heat 2–3 inches of oil to 350°F (175°C). Use a thermometer for accuracy.
Shape the doughnuts. On a lightly floured surface, pat or roll the dough to about 1/2 inch thick. Cut with a 3-inch doughnut cutter (or use a 3-inch round cutter plus a 1-inch cutter for the holes). Gather scraps and re-roll as needed.
Fry in batches. Carefully add 3–4 doughnuts to the oil at a time. Fry 1–2 minutes per side until deep golden brown. Keep the oil between 340–360°F for even cooking.
Drain and coat. Transfer doughnuts to a wire rack set over a sheet pan. While still warm, brush lightly with melted butter, then roll in the cinnamon sugar to coat all sides.
Fry the holes. Cook doughnut holes for about 1–1 1/2 minutes total, turning once. Coat the same way.
Serve. Enjoy warm for the best texture and aroma. They’re fantastic with coffee or cold milk.
