Prepare the rhubarb by putting it in a saucepan with the sugar and 1/4-cup/60 ml water. Bring to a simmer and cook until soft. If it seems too runny, simply leave on the heat, stirring, to let some of the liquid evaporate. Remove the pan from the heat and set aside.
Beat the egg white to stiff peaks with the pinch of salt. Heat the sugar with 3 tablespoons of water to thread stage. Cook's Note: Which is just before it starts to color.
With the beaters going, pour the sugar-water mixture into the beaten egg whites and continue beating until the stiff meringue is cooled. Meanwhile, beat the cream to peaks with the vanilla extract. Fold the two mixtures together.
Spoon the rhubarb in the bottoms of 4 coupes. Top with parfait. Cover and refrigerate for a couple of hours before serving.