01 - Set oven to 350°F. Grease two standard donut pans thoroughly.
02 - Simmer apple cider in a saucepan over medium heat until reduced to 1/2 cup, about 10 minutes. Let it cool slightly.
03 - Whisk together flour, baking powder, baking soda, cinnamon, nutmeg, allspice, and salt in a large bowl.
04 - In a separate bowl, whisk sugar, eggs, melted butter, whole milk, vanilla extract, and cooled reduced apple cider until smooth.
05 - Fold wet ingredients gently into dry mixture until just blended, careful not to overmix.
06 - Transfer batter into a piping bag or zip-top bag and pipe evenly into donut cavities, filling each about three-quarters full.
07 - Bake donuts for 10 to 12 minutes until a toothpick inserted comes out clean.
08 - Allow donuts to cool in pans for 5 minutes before transferring them to a wire rack.
09 - Mix granulated sugar and cinnamon in a shallow dish. Brush each donut with melted butter, then dip into cinnamon-sugar to coat.
10 - Offer donuts warm or at room temperature for optimal flavor.