In a large bowl, mash the bananas and butter well with a fork. Small lumps are okay.
Add the eggs, vanilla extract and lemon juice and mix well.
Add the coconut flour and salt and mix well.
Add the blueberries and gently mix them in without popping them.
Pour the batter into a 8x8 pan and place it in the middle of the oven. Bake for 20 to 25 minutes, until the very top middle is firm to touch. Cut into nine equal squares and serve warm, room temperature, or cold.