Step 1: Reduce the Apple Cider. Pour the apple cider into a small saucepan over medium heat. Simmer the cider for about 15 to 20 minutes until it has reduced to exactly ½ cup. This concentrates the flavor. Set it aside to cool completely before using.
Step 2: Preparation and Preheating. Preheat your oven to 350°F (175°C). Lightly grease two standard 6-cavity donut pans with non-stick cooking spray or a light coating of butter to ensure the donuts release easily after baking.
Step 3: Whisk Dry Ingredients. In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Ensure there are no large lumps and the spices are evenly distributed.
Step 4: Combine Wet Ingredients. In a separate medium bowl, whisk together the melted butter, brown sugar, and granulated sugar. Add the egg, buttermilk, vanilla extract, and the cooled, reduced apple cider. Whisk until the mixture is smooth and well combined.
Step 5: Mix the Batter. Pour the wet ingredients into the dry ingredients. Use a spatula to gently fold the mixture together until just combined. Be careful not to overmix, as this can make the donuts tough; a few small lumps are perfectly fine.
Step 6: Fill the Donut Pans. Spoon the batter into the prepared donut pans, filling each cavity about 2/3 or 3/4 full. For a cleaner look, you can transfer the batter to a piping bag or a large zip-top bag with the corner snipped off to pipe the batter into the rings.
Step 7: Bake the Donuts. Place the pans in the oven and bake for 10 to 12 minutes, or until the donuts spring back when lightly touched and a toothpick inserted comes out clean. Let them cool in the pan for 5 minutes before transferring to a wire rack.
Step 8: Prepare the Maple Glaze. In a small bowl, whisk together the powdered sugar, maple syrup, and 1 tablespoon of milk. If the glaze is too thick, add more milk a teaspoon at a time until it reaches a dippable consistency.
Step 9: Glaze and Set. While the donuts are still slightly warm, dip the top of each donut into the maple glaze. Place them back on the wire rack and allow the glaze to set for about 10 minutes before serving.