You will need a paper lined round tin- NO removable bottom. 18 cm / or 7 inches. Set the oven to 180 C or 350 FLine the bottom of the cake tin with paper that just fits into the bottom to protect the caramel and bananas. Grease or spray the sides.
Make the caramel by putting the brown sugar into a pot with the butter and water. Simmer on high till reduced (by about 1/2) and shiny.
Cut the bananas in half length ways . Pour the caramel into the cake tin and place the bananas on top. Set aside
Make the chocolate cake in a medium bowl adding the flour, cocoa, baking powder, sugar and a pinch of salt mix together well with a whisk to break up any lumps. Add the beaten egg, milk and oil together to combine well in a jug or bowl and then mix into the flour mixture with a whisk till smooth and well combined
Pour the batter over the bananas and caramel.
Bake for 50 minutes till risen and cooked through. (it will puff up a bit)
Cool for 5 minutes and press down in the middle of the cake to flatten. You will be turning the cake over, so a big bulge in the middle will mean the cake wont sit flat (you can trim the top a bit if it has risen into a point). Cool for another 5 minutes. The cake should be just cool enough to handle but not so much that the caramel sets, making it impossible to remove
Run a knife around the edge of the cake tin to loosen it from the sides. You need to turn it out while it is warm or the caramel may set and it will stick.
Put a tray or large plate on top of the cake and invert, giving a little shake (being careful of the caramel). The cake will still be hot.
Top with the extra caramel (if you have any)
Eat it hot with ice cream or cream ( or chocolate sauce). It can also be made ahead and reheated. It will also keep for several days