Preheat the air fryer to 350F.

Prepare the cinnamon crumb topping. In a medium bowl, combine all of the ingredients and mix with a fork until a crumbly mixture forms. Set aside.

Make the sourdough banana donuts. In a large bowl, combine the flour, baking powder, baking soda, cinnamon, salt, greek yogurt, sourdough discard, and mashed banana. Mix until well combined. The dough is going to be sticky, but that's okay!

Lay the dough out onto a generously floured surface. Sprinkle some more flour on top of the dough. Use your hands to knead a couple of times, until the dough becomes smooth. If the dough still seems too sticky, add 1 more tbsp of flour at a time, and continue to knead until it comes together more!

Roll the dough out to ½" thickness. Using a small round cookie cutter, mason jar lid, or cup, cut out mini circles. Then using a smaller round cutter, pop the inner circle hole out of each to create the donut shape.

Place a piece of parchment paper in the basket of the air fryer. Working in batches, transfer the donuts to the air fryer.

Brush the tops with melted butter, then sprinkle with the cinnamon crumb.

Air fry at 350F for 3 minutes, gently flip and air fry for another 3-4 minutes, or until golden brown.

Make the salted caramel drizzle by combining all of the ingredients in a small bowl, and mixing until well combined.

Remove the donuts from the air fryer, and drizzle with the salted caramel. Serve warm.