For the cake base, make a sponge mixture from the above ingredients. Divide the dough into 2 halves and spread each on a springform base lined with baking paper.
For the meringue layer, whip the egg whites with salt until stiff and fold in the sugar. Divide in half as well. Spread one half of the mixture on each cake base and sprinkle with 2 tablespoons of flaked almonds.
Bake the pastry cases at 170 °C (top and bottom heat) for about 30 minutes until golden brown. Let cool completely at room temperature.
For the rhubarb filling, briefly boil the rhubarb pieces with sugar in water until soft. Mix the pudding powder with 6 tablespoons water and 1 tablespoon sugar until smooth and stir into the simmering rhubarb compote. Finish cooking the mixture like a conventional pudding and allow to cool.
For the cream filling, whip the whipped cream with cream stiffener and vanilla sugar until stiff.
Place one of the cooled layers on a cake plate. First spread the rhubarb mixture on top and then the cream filling. Cut the second cake layer into 12 pieces and place them on top of the filling.
Leave the cake to rest in the refrigerator for some time.