Empty custard into a bowl mix in vanilla extract to your desired taste.
Into the custard add the tin of condensed milk and mix together.
In a separate bowl whisk the double cream until it is stiff peaks.
Fold the double cream into the custard mixture.
In a dish put a tiny amount of mixture at the bottom.
Put some of the sponge biscuits on top the first layer with some sliced bananas.
Add a more generous layer of custard mixture on top.
keep repeating steps 6 and 7 until all the mixture has gone.
Finally crumble some more of the sponge biscuits on top for decoration.
Leave in the fridge over night and then serve.