Let the puff pastry rest at room temperature for about 10 minutes. Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Remove the dough from the package, unroll it and place it on a baking tray with the baking paper.
Bake in a hot oven for about 15 minutes. Remove and let cool on a cake rack.
Meanwhile, for the rhubarb sauce, clean, wash and cut the rhubarb into small pieces. Boil up the rhubarb nectar and 2 tablespoons of sugar. Stir 1 tsp. starch and 1 tbsp. water until smooth. Stir into the boiling nectar and simmer for about 1 minute while stirring.
Add the rhubarb pieces, bring to the boil again and continue cooking for 2-3 minutes. Sweeten if necessary, then let cool off.
Cut the baked puff pastry sheet in half crosswise. Place one half on a board. Fold a piece of aluminium foil (at least 90 cm long) several times lengthwise to form a strip about 6 cm high. Place aluminium strips around the puff pastry sheet as a stable frame.
Cut the second puff pastry sheet into 8 equally sized pieces (approx. 6 x 10 cm).
For the vanilla cream, stir 50 g starch and 1⁄8 l milk until smooth. Whisk the egg yolks. Bring the remaining milk, cream and brown sugar to the boil in a large saucepan. Remove from the heat, first stir in the starch, then the egg yolks.
Place back on the stove, bring to the boil while stirring and simmer for 1 minute. Season to taste with vanilla flavouring. Remove from the heat.
Spread the cream on the puff pastry in the aluminium frame and smooth it down. Place the puff pastry pieces close together. Leave to cool, chill for about 3 hours.
Take it out of the fridge and dust it thickly with icing sugar. Cut along the puff pastry pieces into 8 slices. Add the rhubarb sauce.