Simmer the rhubarb with the sugar and water, covered, for approx. 5 mins. until almost soft. Add the strawberries, remove the pan from the heat, allow to cool.
Biscuit dough: Stir the egg yolks, sugar and water for approx. 5 mins., until the mixture becomes paler in colour. Beat the egg whites with the salt until stiff. Layer the flour and the egg whites on the mixture and carefully fold in using a rubber spatula. Spread the mixture onto an oven tray lined with baking paper to a size of approx. 30 x 30 cm.
Bake for approx. 5 mins. in the centre of an oven preheated to 220°C. Remove, turn out onto a sheet of baking paper, carefully peel off the paper. Cover the biscuit with the upturned tray, leave to cool. Using a round cutter the same size as the glass, cut out 18 circles. Combine the milk and all the ingredients up to and including the eggs and bring to the boil over a medium heat, stirring constantly. As soon as the mixture thickens, remove the pan from the heat, stir for about another 2 mins. Pour the crème through a sieve into a bowl, leave to cool. Arrange the biscuit, compote and vanilla crème in layers in the glasses and chill for about an hour.