For the ice cream, put the condensed milk, cream and crushed cardamom seeds into a bowl. Whip until soft peaks form when the whisk is removed from the bowl, then transfer to a freezer-safe tub and level the top. Freeze.
For the tarte tatin, preheat the oven to 180C/160C Fan/Gas 4 and have a 23cm/9in ovenproof pan at the ready.
Roll out the pastry to a 1cm/½in thickness, then cut out a circle about 2cm/¾in wider than your pan. Pop it on a baking tray and leave it in the fridge while you make the caramel.
Put the sugar and butter into the pan, and when the sugar dissolves, turn up the heat and allow the caramel to get dark and thicken. Occasionally lift the pan and give it all a little swirl. The main thing is to watch it until it is a deep caramel colour, because once it gets to that point it can burn quickly.
Remove from the heat and sprinkle on the hazelnuts. Add the bananas to the caramel, then lay the pastry on top and gently lift and tuck it around the sides of the pan, being careful of the hot caramel. Cut a slit in the top for the steam to escape, and bake for 25–30 minutes.
Allow the tart to cool for a few minutes, then place a plate over the pastry and carefully turn out the tart tatin (wear oven gloves to protect your hands from the hot caramel). Serve in slices with scoops of the ice cream.