For the rhubarb vanilla cake, mix soft butter with sugar, salt, vanilla sugar and eggs to a nice mousse, add lemon zest and mix in flour mixed with baking powder. Pour batter into a buttered, floured cake pan and bake at 175 degrees for about 20 minutes.
When cool, place on a cake plate and surround with a cake ring. Mix vanilla pudding powder with 2 tablespoons sugar and stir with some juice. Sauté rhubarb cubes in the remaining juice with the remaining sugar, cubes should not fall apart. Add the stirred custard powder to the rhubarb filling cubes and bring to a gentle boil until the custard thickens.
Set aside 16 nice rhubarb cubes on a plate to decorate the cake. Pour the rhubarb mixture onto the cake base and allow to cool. Soak gelatine, heat and dissolve, whip cream until stiff, mix curd with sugar and vanilla sugar. Add gelatin to curd mixture, stir in, then carefully fold in whipped cream. Pour the mixture into the cake ring on top of the cooled rhubarb mixture, chill for a few hours and allow to set.
Prepare cake glaze with juice as described on package, let cool and carefully spread over top mixture. Allow to cool completely, then pipe rosettes of whipped cream over the rhubarb and vanilla cake, decorating each with the reserved diced rhubarb. And the rhubarb-vanilla cake is ready.