Reduce the apple cider by simmering it on low heat until you have 1/2 cup (120ml). Stir occasionally and check every 10-20 minutes. Set aside to cool.
Preheat the oven to 350°F (177°C) and spray a donut pan with non-stick spray.
In a large bowl, whisk together the flour, baking soda, baking powder, cinnamon, apple pie spice, and salt. Set aside.
Whisk the melted butter, egg, brown sugar, granulated sugar, milk, and vanilla. Add to the dry ingredients along with the reduced apple cider, and mix until smooth.
Spoon or pipe the batter into the donut pan, filling each cavity halfway.
Bake for 10-11 minutes, until the donuts are lightly browned and bounce back when touched. Cool for 2 minutes, then transfer to a wire rack.
Combine the sugar, cinnamon, and apple pie spice. Coat each donut in melted butter, then in the spiced sugar mixture. Enjoy!