Preheat oven to 375°F and spray a donut pan with cooking spray. Set pan aside.
In a large mixing bowl, add the flour, cornstarch, baking powder, cinnamon, salt, and nutmeg. Whisk together to combine well.
In another mixing bowl, add the buttermilk, brown sugar, granulated sugar, egg, melted butter, vanilla and cooled apple cider reduction. Whisk to combine well.
Pour the milk mixture into the large mixing bowl with the flour mixture. Stir until just combined and no flour streaks remain.
Transfer batter to a piping bag, or large zip top bag. Cut about 1/2 an inch off of the corner of the bag.
Pipe the batter into the donut cavities of the prepared donut pan, filling the cavities about halfway.
Bake in preheated oven for 8-10 minutes, until a toothpick inserted in the donut comes out clean or with a few moist crumbs. Another way to tell if they are done is to lightly touch the top of a donut. If it springs back, it's done.
Transfer baked donuts to a wire cooling rack. Re-grease donut pan and repeat with any remaining batter.