Preheat your oven to 180°C (356°F) or prepare your air fryer for baking.
Mash the bananas in a bowl using a fork until smooth.
Add cocoa powder, baking powder and vanilla extract. Mix until well combined.
Fold in the rolled oats, then stir in the chopped dark chocolate and nuts.
If using, mix in dried strawberries.
Spoon the mixture into silicone or paper muffin cases, filling each about ¾ full.
Bake in the Air fryer for 15–20 minutes on the bake setting.
Or bake in the preheated oven for 25 minutes.
Let the muffins cool in their cases for a few minutes, then transfer them to a plate to cool completely.