Make the biscuit base first. Melt the butter in a medium-sized pan. Put the biscuits into a food bag and crush with a rolling pin, or pulse them until fine in a food processor.
Tip the biscuit crumbs into the melted butter until evenly coated. Pour the mixture into the centre of a 23cm/9inch loose-bottomed tart tin. Then press the mixture firmly over the base of the tin and up the sides. Transfer the base to the fridge to chill for about 30 minutes, until firm.
While the base cools, make the caramel. Heat the brown sugar and butter together in a pan over a medium heat until the butter melts and the sugar has dissolved. At first the sugar and butter won’t mix, but keep stirring. As the sugar melts, it will come together.
Stir in the condensed milk and the sea salt flakes. Bring up to the boil, then take off the heat.
Take the set biscuit base out of the fridge and pour the caramel onto the biscuit base. Transfer to the fridge and leave to cool completely.
When ready to serve, slice the bananas and layer them over the set caramel. Pour the cream into a large bowl, and whip with electric hand beaters till thickened but not stiff. Spoon the cream over the bananas.
Grate the chocolate over the top of the pie.
Remove the pie from the tart tin, by placing the tin on top of an upturned bowl, then sliding the edge of the tin down. Move the bowl out of the way, then slide the banoffee pie off the tin base and onto a board or plate. It’s now ready to serve.