First up, preheat your oven to 200°C/180°C/Gas Mark 6. Line a 12 hole muffin tray with muffin cases.
Mix together the milk and lemon juice and leave to one side to thicken slightly. Next up mash your bananas and set aside.
Pop the flour, Baking Powder, Bicarbonate of Soda and sugar into a large bowl and mix together to combine. Make a well in the centre and add the milk, banana, melted butter, Vanilla Extract and egg and whisk together until just combined. Finally pour in the Chocolate Chunks and stir through the mixture.
Divide the mixture equally between the muffin cases and pop into the oven to bake for 20-25 minutes until golden brown and a skewer popped into the centre comes out clean.
Leave to cool in the tin for 10 minutes and then pop the muffins onto a wire rack to cool completely.