Beat the butter with the sugar until it looks very creamy. You can use a whisk, a handheld mixer or a stand mixer.
Add the mashed bananas and beat again.
Add the eggs and beat again to incorporate. It should look foamy.
Add the vanilla, cinnamon, baking powder and salt and incorporate.
Finish the batter by mixing in the flour. If you were using a whisk, switch to a spatula. Then, add the blueberries and fold them in with a spatula.
Line a 22x11cm/ 9x4in pan with parchment or baking paper. Pour the batter inside and level it out.
Optional: Make a like of thin strips of cold butter along the centre to make it open evenly as it bakes.
Bake the blueberry banana bread in a preheated oven at 180℃/350℉ for 1 hour or until you put a skewer or a small serrated knife at the centre and it comes out clean. If the bread needs a bit longer in the oven but it' golden enough on top. you can cover it with aluminum foil.