Preheat the air fryer to 350°F.
Separate biscuit dough and cut a hole in the center of each to create a doughnut shape.
Place doughnuts in the air fryer basket in a single layer and cook for 5-6 minutes or until golden brown.
While they cook, mix sugar and cinnamon in a bowl.
Brush cooked doughnuts with melted butter and coat with cinnamon sugar.