Line a 20 cm square baking tray with baking paper, and heat the oven to 170C/150C fan.
Cream the coconut oil, sugar and banana. Add the flour and form into a dough. Press into the lined baking tray, smooth with the back of a spoon and prick all over with a fork.
Cook for about 25 minutes until golden brown. Allow to cool in the tin before slicing into squares.