Preheat the oven to 180°C, fan 160°C, gas 4. Put the butter in a 22cm heavy-based ovenproof frying pan or roasting tin over a medium heat. Add the sugar and cinnamon, then allow to melt and bubble for a minute until you have a lovely thick sauce.
Arrange the bananas, cut side down, in the pan or tin and set aside.
Roll out the pastry to fit the pan or tin and arrange over the top of the bananas, tucking it in around them. Place in the oven to bake for 25 minutes, until the pastry is golden and cooked through.
Remove from the oven and carefully place a plate over the tart. Quickly but carefully turn it upside down, spooning any sauce left behind on top (you have to do this quickly as the sauce will set fast as it cools).
Serve cut into wedges.
Cook's tip: for babies, simply mash uncooked bananas with a little natural yoghurt and a pinch of cinnamon.