Open the can of biscuits and separate them onto a clean work surface. Use a small round 1-inch cookie cutter or the cap of a plastic bottle to cut a hole out of the center of each biscuit. Save the centers to make donut holes later!
In a small shallow bowl, whisk together the granulated sugar and ground cinnamon until well combined. In a separate small bowl, melt the butter in the microwave. Set both bowls aside so they are ready as soon as the donuts come out of the air fryer.
Preheat your air fryer to 350°F (175°C). Lightly spray the air fryer basket with cooking spray. Place the donuts in the basket in a single layer, ensuring they do not touch. Cook for 3 minutes, then flip them over and cook for another 2 to 3 minutes until they are golden brown.
While the donuts are still hot, quickly dip each side into the melted butter. Immediately toss the buttered donut in the cinnamon-sugar mixture until it is fully coated. Repeat with the remaining donuts and serve immediately.