Clean, wash, drain well and cut rhubarb into slices. Mix a tablespoon of flour into the rhubarb. Cream soft fat, sugar, vanilla sugar and salt.
Alternately add eggs and about 4 tablespoons of flour. Add cream. Mix remaining flour and baking powder and stir in briefly. Finally add rhubarb to the dough. Pour the dough into a greased box form (30 cm length/ 2 litres capacity) sprinkled with breadcrumbs and smooth it down.
Bake in the preheated oven (electric oven: 175 °C/ gas: level 2) on the 2nd rack from the bottom for 11/2 to 13/4 hours. Let the finished cake rest in the pan for about 15 minutes, then turn out of the pan and let it cool down on a cake rack.
Warm up the redcurrant jelly in a pot and let it cool down a little. Mix icing sugar, lemon juice and 2-3 tablespoons of water to a thick glaze. Spread the top of the cake with the icing. Place jelly with a teaspoon lengthwise on the middle of the cake.
Use a spoon handle to pull through the cast in a loop and allow to dry for 3-4 hours. Cut the cake into slices and arrange on a plate. Decorate with lemon balm and serve with whipped cream to taste.