Heat oven to 180°C. Line a 12 hole muffin tin with muffin cases. Peel the bananas and mash in a bowl.

Mix the flour, baking powder, bicarbonate of soda, salt and brown sugar together in a large mixing bowl. Make a Well in the centre and add buttermilk, egg, olive oil and bananas. Quickly fold the ingredients together until just incorporated, taking care not to overmix. Tip in the blueberries and give the batter one or two stirs.

Spoon the batter into the muffin cases and sprinkle with three demerara sugar. The cases will be quite full. Bake in the oven for about 20-25 minutes. Leave to cool and then transfer.