In a bowl, sprinkle the gelatin over the water and let bloom for 5 minutes. Set aside.
In a saucepan, off the heat, combine 125 ml (1/2 cup) of the sugar with the cornstarch. Stir in the eggs, lemon juice, and butter with a whisk. Bring to a boil, stirring constantly. Add the gelatin and stir until completely dissolved. Pour into a large bowl. Cover with plastic wrap directly on the cream and refrigerate until cooled but still pliable, about 30 minutes.
In a clean bowl, beat the egg whites with an electric mixer until soft peaks form. Gradually add the remaining sugar, beating until stiff peaks form. With a spatula, fold the meringue into the lemon cream.
Pour into six dessert cups or glasses. Cover and refrigerate for about 2 hours.
This lemon mousse is actually a chiboust cream, a mixture of pastry cream and Italian meringue.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.