To prepare the rhubarb, simmer with the sugar and vanilla until soft, stirring occasionally. Puree
and set aside to cool.
To prepare the parfait, whip the egg yolks in a stand mixer fitted with the whisk attachment, or with electric beaters, until thick and foamy. Bring 2/3 cup (130 g) sugar and water up to a boil over high heat. Cook sugar to 270 F (hard ball stage), 7 to 9 minutes. It should not colour. Pouring the hot sugar down the side of the bowl, whisk sugar into whipped egg yolks. Increase speed to high and whip until mixture is thick, pale and doubled in volume, 3 to 4 minutes. Let cool completely to room temperature.
Whip the cream to soft peaks and chill. Whip the egg whites until foamy and gradually add sugar and whip until stiff peaks. Fold the whipped cream into the egg yolk mixture, then fold in the egg whites. Fold in the cooled rhubarb and pour into a loaf pan lined with plastic wrap. Freeze overnight.
To serve, turn parfait out onto plate and slice with a hot, dry knife.