Peel and mash the bananas in a bowl, using a fork.
Mix the flour, baking powder, baking soda, salt, and ½ cup brown sugar together in a large mixing bowl. Make a well in the center and add the buttermilk, egg, olive oil and mashed bananas. Mix the ingredients together until just incorporated, then gently stir in blueberries.
Spoon the batter into 12 paper-lined muffin tins and sprinkle them with 1 tablespoon of brown sugar. Bake until they become golden brown on top – about 20 to 25 minutes. To check if they are ready insert a toothpick into the center of the muffin and if the muffin is ready the toothpick should emerge clean. Cool in the pan for a couple of minutes, then transfer to a rack to cool completely.
