1. Preheat the oven to 350 degrees F and grease two donut trays. Set aside.
2. Bring the apple cider to a boil in a small saucepan. Once boiling, reduce to low heat, whisk occasionally and continue to let simmer until reduced to 120g (½ cup). For me, this took about 20 minutes. Check every five minutes or so. Once finished, pour the apple cider into a separate liquid measuring cup and let stand for at least 10 minutes to cool slightly.
360g (1 1/2 c) apple cider, reduced to 120g (1/2c)
3. In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, cardamom, and salt until combined.
220g (1 1/2c) flour, 4g (1 tsp) baking powder, 6g (1 tsp) baking soda, 2g (1 tsp) cinnamon, 1g (1/4 tsp) nutmeg, 1g (1/4 tsp) cardamom, 3g (1/2 tsp) salt
4. To a bowl of a stand mixer, add the sugars, oil, room temperature egg, warm milk, room temperature sourdough discard, and vanilla extract. Mix on medium speed until smooth. This can also be done by hand with a hand mixer.
100g (1/2c) white sugar, 100g (1/2c) light brown sugar, 50g (1/4c) neutral oil, 1 egg, 60g (1/4c) milk, 120g (1/2c) discard, 4g (1 tsp) vanilla extract
5. Add in the dry ingredients and mix on low just until most of the flour is incorporated, then finish by mixing by hand.
6. Slowly pour in the apple cider and whisk until smooth.
7. Fill the prepared donut trays about ½ way full. You can do so by spooning in the batter, but my preferred method is by filling a piping bag with the batter, cutting off the tip and piping in the batter. You can also do this with a large ziplock bag.
8. Bake for 10-12 minutes or until when poked, the donut bounces back and the edges are lightly browned.
9. Leave the apple cider donuts in the trays for 2 minutes, then remove and allow to cool slightly on a wire rack.
10. In a small bowl, mix the sugar and cinnamon, then while still warm, dip each sourdough donut until covered evenly with cinnamon sugar. Enjoy warm!
200g (1c) white sugar, 6g (1 1/2 tsp) cinnamon