Reduce the Cider: Pour 1 ½ cups fresh apple cider into a small saucepan. Simmer over low heat, stirring occasionally, for about 20 minutes until reduced by two-thirds (yielding roughly ½ cup). Let it cool for 10 minutes.
Preheat & Whisk Dry Ingredients: Heat oven to 350°F. Lightly grease your donut pan. In a medium bowl, whisk together the flour, baking powder, baking soda, ground cinnamon, nutmeg, and kosher salt.
Combine Wet Ingredients: In a separate large bowl, whisk the melted butter, brown sugar, granulated sugar, milk, vanilla extract, egg, and cooled reduced cider until smooth.
Mix the Batter: Gently stir the wet mixture into the dry ingredients just until no dry flour streaks remain. Avoid overmixing to keep the crumb tender.
Fill the Pan: Transfer batter into a piping bag (or zip-top bag with the corner cut) and pipe evenly into the pan cavities, filling each about ½ to ⅔ full.
Bake & Cool: Bake for 10 to 12 minutes, or until the tops spring back when lightly touched. Let cool in the pan for 2 minutes, then transfer to a wire cooling rack.
Glaze: Whisk powdered sugar with reserved reduced cider until thick enough to coat the back of a spoon. Dip or brush onto slightly warm donuts and allow the glaze to set.