01 - Simmer apple cider in a small saucepan over medium heat until reduced to 1/2 cup, about 10 to 15 minutes. Allow to cool completely.
02 - Preheat the oven to 350°F. Grease two standard donut pans thoroughly.
03 - In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt until evenly blended.
04 - In a separate bowl, whisk eggs, whole milk, melted and cooled butter, reduced apple cider, and vanilla extract until the mixture is smooth.
05 - Gently fold the wet ingredients into the dry ingredients just until combined; avoid overmixing to maintain tenderness.
06 - Spoon or pipe the batter evenly into the prepared pans, filling each cavity about two-thirds full.
07 - Bake in the preheated oven for 12 to 15 minutes, or until donuts spring back when lightly pressed and a toothpick inserted in the center emerges clean.
08 - Allow the donuts to cool in the pans for 5 minutes before transferring them to a wire rack.
09 - In a shallow bowl, mix granulated sugar and ground cinnamon for the coating.
10 - While the donuts are still slightly warm, brush each with melted butter and immediately toss in the cinnamon sugar mixture to coat evenly.