Step 1: In a small saucepan, simmer the 1.5 cups of apple cider over medium heat for about 15-20 minutes until it reduces to exactly 1/2 cup. Let it cool completely before using.
Step 2: Preheat your oven to 350°F (175°C) and generously grease two 6-cavity donut pans with non-stick spray or butter.
Step 3: In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, cinnamon, nutmeg, cardamom, and salt.
Step 4: In a separate medium bowl, whisk together the cooled apple cider reduction, egg, melted butter, buttermilk, and vanilla extract until well combined.
Step 5: Pour the wet ingredients into the dry ingredients and fold gently with a spatula just until combined. Avoid overmixing to keep the donuts tender.
Step 6: Transfer the batter to a piping bag and pipe into the prepared donut pans, filling each cavity about 3/4 of the way full.
Step 7: Bake for 10-12 minutes or until a toothpick inserted comes out clean. Let the donuts cool in the pan for 5 minutes.
Step 8: Prepare the coating by mixing the sugar and cinnamon in a small bowl. Brush each warm donut with melted butter and roll in the cinnamon-sugar mixture until fully coated.