01 - Pour apple cider into a small saucepan and simmer over medium heat until reduced to 1/4 cup, approximately 10 to 12 minutes. Allow to cool completely.
02 - In a medium mixing bowl, whisk together the flour, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg until well combined.
03 - In a large bowl, whisk the cooled reduced apple cider, melted butter, granulated sugar, light brown sugar, egg, whole milk, and vanilla extract until smooth and homogeneous.
04 - Gradually incorporate the dry ingredients into the wet mixture, stirring gently until just combined. Avoid overmixing to maintain a tender texture.
05 - Set the air fryer to 350°F and preheat. Line the air fryer basket with parchment paper or lightly spray with nonstick cooking spray.
06 - Using a small cookie scoop or floured hands, portion the dough into 1-inch balls, each approximately 1 tablespoon in volume. Arrange them in the air fryer basket with adequate space between each ball.
07 - Air fry the donut holes for 7 to 8 minutes, or until golden brown and a toothpick inserted in the center comes out clean. Cook in batches if necessary.
08 - While still warm, brush each donut hole with melted butter and roll in a mixture of granulated sugar and ground cinnamon, prepared in a small bowl.
09 - Serve the donut holes warm or at room temperature for optimal flavor and texture.