Preheat air fryer at 350°F for 3-4 minutes. Meanwhile, mix the sugar and cinnamon in a shallow bowl. Set aside.
Take biscuit dough out of the can and use a small round cookie cutter or the back of a pastry tip to cut out holes in the middle of each donut. Use the center dough for donut holes.
Spray the interior of the air fryer basket with oil spray. Place donuts In a single layer. Make sure donuts do NOT touch each other or overlap; they will stick to each other if they do.
Air fry at 360°F for 5 minutes. During this time, you can melt the butter and have it ready.
When done, the biscuit tops will be golden brown. Carefully remove the hot donuts (use tongs to avoid burning your fingers if they are too hot) from the air fryer and dredge immediately Into the shallow bowl with cinnamon sugar mix. Be sure to coat all sides.
As you work on the donuts, you might want to first brush them with the melted butter before coating with the cinnamon sugar.
Enjoy immediately!
Notes
The melted butter may not be necessary, as the heat from the freshly air fried donuts will often cause the cinnamon sugar to coat the donuts without butter. However, you may find that as you continue to work through coating the donuts, they start to cool and dry, and that is when you can brush all sides with melted butter before dredging them in the cinnamon sugar. You can also use the melted butter from the start if you wish.