Peel and finely grate the sweet potato. Mix flour, 150 g sugar, baking powder, baking soda, salt, vanilla sugar and cinnamon. Peel banana and puree pulp with oil and yoghurt. Add eggs, banana puree to the flour mixture and stir in briefly. Fold in grated potatoes.
Put the dough into a greased, flour-sprinkled box form (approx. 30 cm long) and smooth it down. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 50 minutes. Remove from the oven, leave to rest in the mould for approx. 5 minutes, then turn out. Place the cake on a cake rack and let it cool down.
Coarsely chop the pumpkin seeds. In a frying pan without fat while turning lightly roast. Sprinkle 2 tablespoons of sugar over it, let it caramelize. Add 4 tablespoons of water, remove from heat and allow to cool. Whip cream cheese and icing sugar with the whisk of the hand mixer. Spread the cream on the cake. Spread pumpkin seeds on top.