Clean, wash and cut the rhubarb into large pieces. drain
Cream fat, 1 pinch of salt, vanillin sugar and 200 g sugar with the whisks of the hand mixer. Separate the eggs. Stir in 3 portions of egg yolks. Mix flour and baking powder. Add alternately with buttermilk and stir in
Put the dough on a greased fat pan and smooth it down. Spread rhubarb evenly on top. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 25 minutes
Cut the vanilla pod lengthwise. Scrape out vanilla pulp with the back of a knife. Whisk egg white, salt and vinegar with the whisk of the hand mixer until stiff, add 350 g sugar. Add vanilla pulp. Beat until the sugar is completely dissolved and the meringue is shiny white
Take the cake out of the oven, spread the meringue loosely on the cake, leaving a margin of about 1 cm all around. Bake the cake at the same temperature for another 15-20 minutes
Remove from the oven, place on a cake rack and allow to cool. Dust with icing sugar. Cut open, arrange on plates and decorate with lemon balm as desired