01 - Simmer apple cider over medium heat until reduced to approximately 1/2 cup, about 10 to 15 minutes. Allow to cool to room temperature.
02 - Preheat oven to 350°F. Grease a donut pan thoroughly.
03 - Whisk together flour, baking powder, baking soda, cinnamon, nutmeg, allspice, and salt in a medium bowl.
04 - In a large bowl, whisk melted butter with granulated sugar and brown sugar until well combined. Add eggs one at a time, whisking after each addition. Stir in cooled apple cider reduction, milk, and vanilla extract.
05 - Gently fold dry ingredients into the wet mixture until just combined. Avoid overmixing to maintain tender texture.
06 - Spoon or pipe the batter into the prepared donut cavities, filling each about three-quarters full.
07 - Bake for 12 to 15 minutes until donuts spring back lightly when touched and a toothpick inserted in the center comes out clean.
08 - Let donuts rest in the pan for 5 minutes before transferring to a wire rack to cool completely.
09 - Combine sugar and cinnamon in a bowl. Brush donuts with melted butter on both sides, then dip into the cinnamon-sugar mixture until coated evenly.
10 - Serve donuts warm or at room temperature.