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.