Preheat oven to 180C/170C fan.
Line muffin tray with paper liners.
Melt the butter either in the microwave or in a pan on a low heat. Leave it to cool a little
Mix the flour, sugar, baking powder, bicarb and salt in a large mixing bowl
Add the mashed banana, vanilla and egg into the melted butter and stir until mixed.
Stir the wet ingredients into the dry mixture and beat until mixed. If the batter is a bit dry and stiff, add a little milk and stir in to ease the batter
Stir in the chocolate chips.
Spoon the batter into the muffin cases to 3/4 full. Depending on the size of your cases you may get 9-12.
Bake in the oven for 20-25 minutes until a toothpick can be stuck in coming out clean. Cool on a wire rack.
You can easily swap the brown sugar for caster or demerara as you prefer.