In a mixing bowl, combine the all-purpose flour, sugar, baking powder, and salt. Mix well to combine.
In another bowl, whisk together the milk, egg, melted butter, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Be careful not to overmix; a few lumps are okay.
Preheat your air fryer to 350°F (175°C) for about 3 minutes.
Lightly grease the air fryer basket with non-stick spray or olive oil to prevent sticking.
Using a small muffin scoop or spoon, drop the batter into the air fryer basket, making small round dollops (about 1 tablespoon each). Leave space between them as they will expand.
Air fry the donut bites for 6-8 minutes, or until they are golden brown and cooked through. You may need to work in batches depending on your air fryer size.
Once done, remove the donut bites from the basket and let them cool for a minute.
In a small bowl, combine powdered sugar with cinnamon (if using). While the donut bites are still warm, roll them in the sugar mixture until fully coated.
Notes
For extra flavor, add cinnamon to the powdered sugar.