Preheat oven to 375 degrees Fahrenheit. Spray two 6-count donut pans with cooking spray.
Pour apple cider into a saucepan. Simmer/gently boil until cider is reduced to 1/2 cup. This takes approximately 30 minutes. If it reduces too much, add extra cider to make 1/2 cup. Set aside to cool slightly.
Whisk together the flour, baking powder, baking soda, salt, apple pie spice, and cinnamon in a large bowl. Set aside.
In a separate large bowl, whisk together the sugar, brown sugar, milk*, butter, and egg.
Add the apple cider reduction and the dry ingredients to the bowl of wet ingredients. Whisk until just incorporated. Do not over-mix.
Spoon or pipe the batter into the prepared donut pans, filling them 2/3 to 3/4 full. TIP: Put batter into a gallon size zip-top bag and cut off the corner for piping.
Bake at 375˚F for 9-12 minutes until the tops are lightly browned and the cake springs back slightly to the touch. Let donuts cool slightly for 3-5 minutes, then remove donuts onto a silicone lined baking sheet. Cut out any donut holes as necessary.
Combine granulated sugar, cinnamon, and apple pie spice into a large shallow bowl or square baking dish. While still warm, brush the donuts lightly with melted butter. Coat donuts in the cinnamon-sugar mixture. Enjoy warm.
Notes
Apple Pie Spice - Mix together 2 1/4 tsp ground cinnamon, 1/4 tsp ground nutmeg, and 1/4 tsp ground allspice to have enough apple pie spice for the cake batter and coating.
Nuke milk for 15-20 seconds in the microwave to warm it up. Aim for 100-110˚F.
If you have leftover batter, bake it into muffins at 375˚F for about 14 minutes.
Store donuts at room temperature in an airtight container up to 3 days. Feel free to nuke them in the microwave for 10 seconds if you want to re-heat them.