Grease 9" x 9" ovenproof dish with butter. Lay half of the butter cookies or graham crackers at the bottom of the pan. Set aside.
In a saucepan, dissolve gelatin with milk, water and eggs. Put over medium heat, stirring constantly until mixture thickens. Add vanilla extract. Pour half flan mixture over cookies. Let cool till half set.
Layer up the bananas on top of flan. Pour remaining flan. Top with remaining cookies or Graham crackers.
Press them lightly onto the flan. Chill and serve.