To make the caramel, peel the label off the can of condensed milk and put on its side in a deep pan.
Cover with boiling water - you want the can at least about an inch under water at all times, bring to a simmer and leave for 2hours, checking and topping up the water level regularly. Don't let the can peek out of the water as it may explode.
After 2 hours, drain, carefully remove the can from the pan and leave to cool - it will take a good 2hours or more. Don't try to open the can while still hot - again, it may explode from the pressure.
If you want to make your own pastry pie case, recipe as follows otherwise skip to next step: - mix 1 ½ cups/200g all purpose/plain flour with 1 stick/4oz/110g unsalted butter, cold and cut into chunks, and 2tbsp confectioners/icing sugar, in a food processor until like breadcrumbs. While pulsing the mixer, slowly add some cold water until the dough just comes together (around 2-3tbsp). Form dough into a ball, cover in cling film and chill for 30mins before rolling out. Preheat oven to 375F/190C. Roll out dough and line a greased pie dish or tin. Press gently into dish to ensure there are no air bubbles underneath, trim the edges and prick the base a few times with a fork. Bake blind (ie filled with parchment paper and beans/baking weights) for around 10-15min, remove beans and bake 'open' for a further 15-20minutes until lightly brown and firm to touch. Remove and allow to cool.
Open the cooled can of condensed milk and fill the crumb/pastry base with a layer of the caramel - thickness to your liking but you want a decent layer, using probably half the can. Up to this point can be done a little while in advance.
Slice the bananas in medium-thick round slices, toss with a couple squeezes of lemon juice -only enough to lightly coat, not make them wet - then layer on top of the caramel (you can do thinner slices and put on a couple layers, if you prefer).
Whip the cream, with the vanilla if using, until it forms soft peaks and then layer it on top of the bananas and smooth the surface (you can make stiffer and pipe it on, if you prefer).
Top with a few shavings or gratings of dark chocolate and serve, or refrigerate until needed.