The day before, simmer the condensed milk can in plenty of water for 3-4 hours, adding water several times. Take the can out and put it in a cool place overnight.
Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Bring 100 ml water to the boil in a small pot, stir in baking cocoa. Coarsely chop the biscuits, except for 2. Line a small springform pan (20 cm Ø) with baking paper.
For the chocolate cake, whisk butter, 150 g white sugar, brown sugar and vanilla sugar for about 6 minutes until creamy. Stir in the eggs one by one. Stir in flour and cocoa mixture. Peel banana and mash with a fork. Stir banana and biscuit crumbs into the dough. Fill into the tin, smooth it down. Bake in a hot oven for 30-40 minutes. Take out the cake, let it cool down, put it in a cool place for about 1 hour.
Spread the condensed milk caramelised in the tin on the cake and refrigerate for another 30 minutes.
In the meantime, whisk the sour cream and the remaining 2 tbsp. sugar with the whisks of the mixer for about 5 minutes. Pour into a piping bag with a perforated spout and squirt small, high spots onto the pie. Dust with baking cocoa. Crumble the remaining biscuits and sprinkle over them.
Tip: Eat the rest of Banoffee-Pie the next day. Therefore it is best to decorate only one half with spots and spray the rest of the tuffs freshly onto it.