Weigh all the ingredients. Sieve the flour into a medium mixing bowl. Add baking powder, salt and sugar and mix well.
Add the butter and cut it into the flour using a pastry cutter (or a large firm fork or fingertips), until the mixture roughly resembles a crumb texture, if there are visible large butter pieces left, use your fingertips to break/rub them into the flour mixture. Make a well in the centre.
Mash the bananas, and set aside.
Whisk together the buttermilk, eggs and vanilla extract. Pour in the well of the flour mixture, add mashed bananas and combine everything. Using a wooden spoon.
Coat blueberries with little flour and add in the batter, carefully fold with a spatula.
Cover the batter and let it rest for 1 hour. Resting the batter for at least an hour, will help the muffins to rise and dome.
Preheat the oven to 180℃.
Coat muffin pans with baking spray.
Add the batter into the muffin pans about 2/3 of the batter for each muffin. I used half a cup for large/jumbo muffins.
Add more blueberries on top, sprinkle the tops with brown sugar for a nice crunchy top.
Bake the muffins for 30 to 40 minutes or until a toothpick comes out clean when inserted into the centre.
Let the muffins cool in the pan for 10 minutes and then carefully remove and allow to cool on a cooling rack.