01 - Preheat the oven to 350°F and grease a standard donut pan.
02 - Simmer apple cider in a small saucepan over medium heat until reduced to 1/2 cup, about 7 to 10 minutes, then let cool.
03 - Whisk together flour, granulated sugar, brown sugar, baking powder, baking soda, cinnamon, nutmeg, allspice, and salt in a large bowl.
04 - Whisk reduced apple cider, melted butter, milk, eggs, and vanilla extract in a separate bowl until smooth.
05 - Gently fold wet ingredients into dry ingredients until just combined, avoiding overmixing.
06 - Spoon or pipe the batter into the prepared donut pan cavities, filling each about three-quarters full.
07 - Bake for 12 to 15 minutes until donuts are golden and a toothpick inserted comes out clean.
08 - Allow donuts to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
09 - Brush each donut with melted butter, then dip into the cinnamon-sugar mixture to evenly coat.