Put the ring of a springform pan (24 cm ⌀) on a cake plate and put the sponge base inside it.
Soak 4 sheets of white gelatine in cold water. Mix the yoghurt with the sugar. Transfer the gelatine, dripping wet, to a small pan and dissolve over a low heat. Mix with a little yoghurt then fold into the rest of the yoghurt. Spread the mixture on the cake base and put into the fridge to set (2 - 3 hours).
Trim and peel the rhubarb. Reserve 1 stick for decoration and cut the rest into pieces. Put into a pan with the wine and sugar and simmer for 10 minutes. Soak the red and white gelatine in cold water and stir into the hot rhubarb. Cool, stirring. Spread the mixture on top of the yoghurt layer and put into the fridge to set (4 hours).
Whip the cream stiffly and transfer to a piping bag. Remove the springform ring and decorate the cake with cream rosettes and pieces of rhubarb. Sprinkle with grated chocolate.