Preheat the oven to 180°C (or set your air fryer to 180°C).
Mash the bananas in a large bowl using a fork or potato masher until mostly smooth.
Add the eggs and mix well.
Stir in the flour, oats, baking powder, mixed spice and cinnamon until fully combined.
Gently fold in the chopped chestnuts and chocolate.
Divide the batter evenly between 12 silicone muffin cases or a lightly greased muffin tin.
Bake for 20–25 minutes, until golden and a skewer inserted into the centre comes out clean.
Allow the muffins to cool slightly before removing from the cases.