Preheat the air fryer. Set it to 350°F (175°C) for at least 3–4 minutes. A hot basket helps the donuts puff and brown evenly.
Mix the dry ingredients. In a medium bowl, whisk flour, sugar, baking powder, salt, and cinnamon (if using) until combined.
Combine the wet ingredients. In another bowl, whisk milk, egg, melted butter, and vanilla until smooth.
Make the batter. Pour wet ingredients into dry.
Stir gently until just combined. Do not overmix; a few small lumps are fine.
Prep the air fryer basket. Lightly mist the basket with oil spray. Avoid heavy spraying to prevent sticky residue.
Portion the donuts. Transfer batter to a piping bag or a zip-top bag with a small corner cut. Pipe small rings (about 1 1/2 inches wide) onto parchment paper rounds or into silicone donut molds made for the air fryer.
Leave space between each donut.
Air fry in batches. Cook 5–7 minutes, flipping halfway if not using molds. Donuts should be puffed and lightly golden. They’re done when a toothpick comes out clean.
Coat or glaze. For cinnamon sugar, brush donuts with melted butter and toss in the sugar-cinnamon mix while warm. For glaze, whisk powdered sugar, milk, vanilla, and a pinch of salt, then dip cooled donuts and let set.
Serve warm. Enjoy right away for the best texture—soft inside with a delicate crust.