In a large heat-resistant bowl, whisk the egg yolks, lemon zest, lemon curd, lemon juice, granulated sugar and a pinch of salt.
Place the bowl over a pan of boiling water, making sure that the bottom of the bowl does not touch the water. Cooking, stirring constantly, until mixture is thick & creamy, about 10 minutes.
Removing bowl from the heat and stir in vanilla extract. Letting mixture cool completely.
In a separate bowl, whisk the butter until light and thin. Gradually add the powdered sugar and whisk until well combined.
Fold butter mixture to the cooled lemon mix.
In a large bowl, whisk the egg whites until harsh peaks form. Fold egg whites to lemon mixture.
In a separate bowl, whisk the double cream until harsh peaks form. Sweetly fold the whipped cream to lemon mix.
Pour the mousse into a serving dish or individual cups and leave in the fridge for at least 2 hours, or until set.
Serve chilled and garnished with extra lemon peel, if desired.