Place biscuits or crackers in a large freezer bag and crush them with a rolling pin until the are fine crumbs. Then combine with the melted butter. Spread the mixture evenly on the bottom of a tart tin and press it firmly with your hands or a spoon. Then put it in the fridge to cool.
For the toffee melt sugar and butter in a pot. This can take a little while, keep an eye on it and stir. As soon as the sugar is completely melted, carefully stir in the condensed milk and let it simmer for a few more minutes. Take the tart out of the fridge and spread toffee on top. Place in the fridge for another hour.
Whip the cream and add cream stiffener then carefully combine with sourcream.
Slice bananas and place on the cooled toffee evenly, then spread the cream on top.
Use a potato peeler or grater to grate chocolate over the pie and serve chilled.