Preheat the oven to 180°C, fan 160°C, gas 4. Line 2 x 12 hole mini-muffin tins with small paper cases. Melt the butter in a small pan and leave to cool slightly. Peel and mash the bananas.
In a large mixing bowl, whisk the milk, sugar, eggs and melted butter.
Sift in the flour and baking powder and add the banana and chocolate. Using a large metal spoon, fold everything together until combined but don’t over-mix.
Divide the mixture between the muffin cases and bake for 25 minutes until risen and golden. Leave to cool on a wire rack.