Do not make this filling until you are ready to use it. You may substitute orange juice for the lemon juice, if desired. If you do so, add 1 tablespoon (45 ml) orange flower water to the juice.
Beat the egg yolks with approximately one-quarter of the sugar to the ribbon stage. Add the lemon juice and whip over a bain-marie until the mixture reaches 140°F (60°C) and is light and foamy. Remove from the heat.
Place the gelatin mixture over the bain-marie and heat until just dissolved. Whisk the softe