Pre-heat the oven to 160C (320F) fan-forced. Line a mini muffin tin with paper cases or grease it well.
In a medium bowl, combine the bananas, butter, egg, yoghurt and vanilla.
In another bowl, combine the sugar, flour, oats, baking powder, cinnamon and salt.
Make a well in the dry ingredients and pour in the wet ingredients.
Gently stir them together until everything is just combined.
Spoon the batter into the muffin tins.
Sprinkle each one with a little muesli.
Bake the muffins for 15-18 minutes or until a tester comes out clean and they are golden on top.
Leave them to rest in the tins for 5 minutes and then transfer to a wire rack to cool completely.