Preheat and prep pans. Preheat the oven to 375oF (190 degrees C). Grease two 6-cup donut pans with non-stick cooking spray.
Make apple cider reduction. Pour apple cider into a saucepan and place over medium heat. Bring to a simmer and let it cook, watching carefully, until the cider is reduced to ½ cup. This took about 40-45 minutes for me. If it reduces too much, add enough cider to make ½ cup. Let it cool for about 10 minutes before using. I pop mine in the freezer to cool down faster. This can also be done the day before. Simply cover and refrigerate until ready to use the next day.
Mix dry ingredients. Add flour, baking powder, baking soda, salt, cinnamon,and nutmeg to a large bowl. Mix with a whisk until combined and set aside until needed.
Mix wet ingredients. Whisk sugar, brown sugar, milk, butter, vanilla extract, and egg together in another bowl until combined.
Combine all ingredients. Make a well in the center of the dry ingredients. Add the apple cider reduction and the wet ingredients to the well. Stir together using a spatula to form a slightly thick batter; do not overmix.
Pipe donuts. Spoon or pipe the batter into the prepared donut pans, filling them about 1/2 of the way up. Or use a piping bag and fill the pans, which is much easier.
Bake donuts. Bake in the center of the preheated oven until the tops are lightly browned, and the donuts spring back slightly to the touch, 10 to 12 minutes. Let cool for 10 minutes in the pans before removing to a sheet pan lined with a silicone baking mat. Pipe and bake any remaining batter with a second batch (I usually get another 3-4 donuts).
Coat donuts. Mix sugar and cinnamon together for topping in a shallow dish. To coat the donuts you can roll them in just cinnamon and sugar. Or dip them in melted butter first then roll in the cinnamon sugar mixture. Let cool completely before serving (if you can wait that long!)