In a small bowl, mix together brown sugar and cinnamon.
Brush the pineapple spears with the melted butter. Sprinkle cinnamon sugar over the spears, pressing lightly to ensure it adheres well.
Place the spears into the air fryer basket in a single layer. Depending on the size of your air fryer, you may have to do this in batches. Set fryer to 400°F for 10 minutes for the first batch (6-8 minutes for the next batch as your air fryer will be preheated). Halfway through, brush with any remaining butter.
Pineapples are done when they are heated through and the sugar is bubbling.