Preheat the oven to 375˚F. Line a 9x9 inch baking pan with parchment paper and set aside.
In a medium bowl, combine the flour, cornstarch and sugar.
In a separate bowl, mix together the butter, banana and the lemon juice. Slowly mix together the dry ingredients until fully incorporated.
Using a rubber spatula, press the cookie dough into the prepared baking pan and bake in the preheated oven for 14-15 minutes, or until golden. Remove and allow to cool completely in the pan before cutting into slices.