01 - Pour apple cider into a small saucepan and simmer over medium heat until reduced to 1/4 cup, about 10 to 12 minutes. Allow cider to cool completely before use.
02 - In a medium mixing bowl, whisk together all-purpose flour, baking powder, baking soda, salt, ground cinnamon, and nutmeg until evenly combined.
03 - In a large bowl, combine cooled reduced apple cider, melted unsalted butter, granulated sugar, brown sugar, egg, whole milk, and vanilla extract. Whisk until the mixture is completely smooth.
04 - Add the flour mixture to the wet ingredients and gently mix until just combined. Avoid overmixing to retain a tender texture.
05 - Set air fryer to 350°F. Line the basket with parchment paper or coat lightly with nonstick spray to prevent sticking.
06 - Use a small cookie scoop or lightly floured hands to form 1-inch balls (approximately 1 tablespoon of dough each). Arrange in the air fryer basket, spacing them apart for even cooking.
07 - Cook for 7 to 8 minutes, or until golden brown and a toothpick inserted into the center comes out clean. Work in batches if necessary to avoid overcrowding.
08 - In a small bowl, mix together granulated sugar and ground cinnamon for the coating.
09 - While donut holes are still warm, brush with melted butter, then roll each one in the cinnamon-sugar mixture until fully coated.
10 - Enjoy donut holes warm or at room temperature. Allow leftovers to cool completely before storing.