01 - Pour apple cider into a small saucepan and bring to a simmer over medium heat. Reduce until approximately 1/2 cup remains, about 10 to 15 minutes. Let cool to room temperature.
02 - Preheat oven to 350°F (175°C). Lightly grease a donut pan to prevent sticking.
03 - Whisk together flour, baking powder, baking soda, cinnamon, nutmeg, allspice, and salt in a medium bowl until evenly mixed.
04 - In a large bowl, whisk melted butter with granulated and brown sugars until 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 wet mixture until just combined; avoid overmixing to maintain tender texture.
06 - Spoon or pipe batter into prepared donut pan cavities, filling approximately three-quarters full.
07 - Bake for 12 to 15 minutes until donuts spring back when lightly pressed and a toothpick inserted comes out clean.
08 - Allow donuts to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
09 - Combine sugar and cinnamon in a shallow bowl. Brush each donut generously with melted butter, then dip both sides into the cinnamon-sugar mixture.
10 - Serve donuts warm or at room temperature for best flavor.