Cut each piece of bread into 9 cubes. 

In a bowl, whisk together egg whites, flour, milk, and vanilla until combined. Dip each cube of bread into mixture, shaking off any excess, and transfer to a large plate. 

Preheat air fryer at 350F. Spray air fryer basket with nonstick cooking spray. Add cubes in a single layer into basket (most likely will need to work in multiple batches). Spray with additional nonstick cooking spray. Air fry at 350F for 5-6 minutes until golden brown. Cool bread for 5 minutes or until safe to touch.

In a shallow bowl, whisk together cinnamon and sugar.  Dip each cube into melted butter then coat in cinnamon-sugar mixture. 

(Optional): serve bites warm with maple syrup.