For the shortcrust pastry, put the flour into a bowl and rub in the butter with your fingertips until the mixture resembles fine breadcrumbs. Add 2-3 tablespoons of cold water gradually, mixing to form a soft dough.
Roll out the dough on a lightly floured work surface and use it to line a 20cm/8in flan tin. Cover in cling film and place in the fridge to chill for 30 minutes.
Preheat the oven to 200C/400F/Gas 6.
Line the pastry case with foil and fill with baking beans. Bake blind in the oven for about 15 minutes, then remove the beans and foil and cook for a further five minutes to dry out the base. Remove from the oven.
For the toffee filling, put the butter and sugar in a large non-stick pan. Heat gently until the butter has melted and the sugar has dissolved, then add the condensed milk. Stir continuously and evenly with a flat-ended wooden spoon for about five minutes, or until the mixture is thick and has turned a golden toffee colour – take care, as it burns easily. Turn the filling into the cooked pastry case and leave to cool and set.
For the topping, whip the double cream in a large bowl until it just holds its shape and spread evenly over the cold toffee mixture. Peel and slice the banana and dip it into a little lemon juice to prevent discolouring.
Pile the banana slices on top of the cream and sprinkle the whole pie with grated chocolate. Remove the ring of the flan and transfer to a flat plate. Serve well chilled.