Preheat the oven to 400 F/ 200 C/ gas mark 6. Lightly grease or line a muffin pan with 12 muffin cases. In a bowl combine the flour, sugar, baking powder and chocolate chips and stir using a whisk. Set aside.
In another bowl whisk together the egg, peanut butter, mashed banana, vanilla, milk and oil until well combined.
Pour in the dry mixture and stir just to combine (the mixture will be lumpy). Spoon the mixture into the muffin cases and bake in the centre of the oven for 23 minutes. Remove from the oven and set aside for 5 minutes then transfer the muffins onto a cooling rack.
Peanut butter: Use either smooth or crunchy.
Muffin Batter: Do not overstir the batter. Stir just to combine. The batter should be lumpy.
Best served on the day. Store in a plastic container for up to 2 days. Pop in a microwave for a few seconds before serving if possible.