Heat the oven to 170C/fan 150C/gas 3½. For the caramel, put the sugar and 25ml water in a 20cm oven and stove-proof dish (a small ovenproof frying pan would also work) on a low heat. Stir the sugar a few times, until it dissolves.
Simmer for around 10 minutes, until the sugar becomes a caramel, and slowly turns to a nutty dark brown colour (it should turn dark reddish brown to get the deep colour on the finished tart). Tip the pan to get an even colour, rather than stirring. Take off the heat and whisk in the butter until smooth.
Peel and thinly slice the bananas then layer in an overlapping circular pattern over the caramel (be careful not to touch it as it will still be hot). The caramel might set a bit, but will melt again in the oven. Sprinkle the cinnamon over the top.
Roll the puff pastry out on a floured worksurface to 2-3 mm thickness and cut the pastry in a round to cover the bananas in the dish. Put the puff pastry lid over the bananas and tuck in the edges with a knife.
Bake for 25-30 minutes, or until the pastry puffs up and turns light brown on top. Leave for 5 minutes for the molten caramel to cool a little.
Very carefully turn the tart out onto a plate with the banana facing up. Sprinkle the banana chips onto the sides of the tart and drizzle the toffee sauce around it, as in the image above. Serve hot, with cinnamon ice cream.