Use a 20-cm springform tin. Peel sweet potatoes and bananas and slice into rounds.
Place the potato and banana slices in a large pan and cover with the milk; top up with water if there is not enough milk. Bring to the boil. Cook slices over low heat for 20 minutes or until cooked and softened.
Drain potato and banana in a sieve. Purée in a blender; set aside. Preheat oven to 200°C.
Slice the vanilla bean in half lengthwise and scrape out the seeds with a sharp knife. In a small bowl, beat the eggs and sugar with electric beaters until the mixture is pale and fluffy.
Add 125 g butter, the rum, the vanilla seeds and the purée to the egg mixture. Stir until combined and smooth.
Grease springform tin with the remaining butter. Place in an ovenproof dish which is larger and a little higher than the springform tin. Pour lukewarm water into the dish, filling it to just over one-third full. Bake cake for 40 minutes.
Remove cake from oven and allow to cool in the tin. Turn out onto a serving platter. Heat jam over a low heat and brush over the cake. Serve immediately.
It's important that the springform tin used to bake this cake in can be relied upon not to leak. If you have doubts, use a soufflé mould.
Used this recipe as a guide more than anything - I had frozen some bananas and roasted sweet potato so used them rather than fresh (and omitted the steps on prepping them). Basically threw all the rest of the ingredients into a blender, whizzed it up and put it into the oven. It came out like a creme caramel, with a lovely flavour. Definitely on the list to do again for guests. - 13 Feb 2016