Simmer 1 1/2 cups fresh apple cider in a saucepan over medium-low heat for 15-20 minutes until reduced to exactly 1/2 cup concentrated syrup. Cool completely.
Preheat oven to 350°F (175°C). Generously grease two 6-cavity standard donut pans with non-stick baking spray.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, nutmeg, and allspice.
In a medium bowl, whisk together the cooled reduced apple cider, melted butter, egg, light brown sugar, granulated sugar, whole milk, and vanilla extract until glossy.
Pour the wet ingredients into the dry bowl. Gently fold with a spatula just until combined without overmixing.
Transfer batter into a piping bag and pipe evenly into donut cavities, filling each about 3/4 full. Bake for 10 to 12 minutes until springy to the touch.
Cool in the pan for 5 minutes, then invert onto a wire rack. While still warm, dip each donut into melted butter, then roll generously in cinnamon sugar.