01 - Simmer apple cider in a small saucepan over medium heat until reduced to about 1/3 cup, approximately 10 to 12 minutes. Allow to cool.
02 - Preheat oven to 350°F. Grease two 6-cavity donut pans thoroughly.
03 - In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, allspice, cloves, and salt until evenly distributed.
04 - In a large bowl, whisk reduced apple cider, granulated sugar, brown sugar, eggs, melted butter, buttermilk, and vanilla extract until smooth.
05 - Gently fold dry ingredients into wet mixture until just combined, taking care not to overmix.
06 - Spoon or pipe batter evenly into prepared pans, filling each cavity approximately three-quarters full.
07 - Bake for 12 to 15 minutes, or until donuts spring back when lightly pressed and a toothpick inserted comes out clean.
08 - Allow donuts to cool in pans for 5 minutes before transferring to a wire rack to cool completely.
09 - Combine sugar and cinnamon in a shallow bowl. Dip warm donuts into melted butter, then coat evenly with cinnamon sugar. Serve warm or at room temperature.