Preheat the oven to 430°f / 220°c. Line a muffin tin with liners of your choice - you can make your own by cutting 5.5” (14cm) squares of parchment. I made seven muffins, if you want them a little smaller you can make 8-9 muffins.

In a medium bowl, sift together the flour, baking powder, sugar, and salt.

In a medium saucepan, melt the butter over low heat until melted, then add the milk, briefly placing it over the heat again if the milk causes the warm melted butter to curdle. Leave to sit for a few minutes to cool slightly. Add the vanilla and mix to combine.

Add the egg and mashed banana to the milk and butter mixture, and whisk with a fork to combine. Add the wet ingredients to the dry ingredients, and mix until some flour still remains, then add the chocolate and mix until just combined.

Divide the batter evenly between 7 holes in the muffin tin using a spoon or scoop (I used a #8 or a ½ cup scoop), then sprinkle with turbinado sugar.

Bake the muffins for 10 minutes at 430°f / 220°c, then turn the temperature down to 375°f / 190°c and bake for another 11-12 minutes until the muffins spring back when touched and a skewer inserted into the middle comes out clean.

Remove from the oven and leave to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Store leftovers in an airtight container at room temperature for up to 4 days. Re warm briefly before eating.