Clean, wash and chop the rhubarb. Caramelise 100 g sugar in a pot. Add rhubarb and 100 ml hot water. Bring to the boil and simmer over a high heat for 6-8 minutes until the rhubarb is soft and almost completely crumbled.
Sweeten the compote if necessary. Let it cool down.
Line a baking tray (approx. 35 x 40 cm) with baking paper. Separate 5 eggs. For the meringue, beat the egg whites until very stiff, gradually adding 1 pinch of salt and 200 g sugar. For the sponge mixture, white-cream egg yolk, 2 eggs and 50 g sugar.
Sift the flour over it and carefully fold it in. Pour both masses into a piping bag with a very large perforated spout.
Preheat oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer). Spray 6 strips of meringue lengthwise onto the baking tray, leaving approx. 2 cm free between each strip. Spray the spaces between the strips with meringue, leaving a gap in the middle of the baking tray.
Bake in a hot oven for 25-30 minutes until the pastry is lightly browned. Let them cool down. Carefully turn meringue biscuit plates over onto a second piece of baking paper. Peel off the baking parchment.
Turn the plates over again using the baking paper.
For the cream, soak gelatine in cold water and weigh 350 g rhubarb compote. Mix with sour cream and syrup. Squeeze the gelatine, dissolve at low heat. Stir 2-3 tablespoons of cream into the gelatine, then stir everything into the rest of the cream.
Cold. Whip the cream until stiff. As soon as the cream starts to gel, fold in the cream.
Immediately spread the cream smooth on a meringue biscuit plate. Place the second meringue biscuit platter on top. Whip the excess baking paper over the cake. Chill for about 2 hours.
Cut the cake into about 10 slices. Dust with icing sugar and serve with the remaining compote.