Instructions:
- In a large heatproof bowl, whisk together the egg yolks, lemon zest, lemon curd, lemon juice, granulated sugar, and a pinch of salt.
- Place the bowl over a saucepan of simmering water, making sure the bottom of the bowl does not touch the water. Cook, stirring constantly, until the mixture is thick and creamy, about 10 minutes.
- Remove the bowl from the heat and stir in the vanilla extract. Let the mixture cool completely.
- In a separate bowl, beat the butter until light and fluffy. Gradually add the powdered sugar and beat until well combined.
- Fold the butter mixture into the cooled lemon mixture.
- In a large bowl, beat the egg whites until stiff peaks form. Fold the egg whites into the lemon mixture.
- In a separate bowl, whip the double cream until stiff peaks form. Gently fold the whipped cream into the lemon mixture.
- Pour the mousse into a serving dish or individual glasses and refrigerate for at least 2 hours, or until set.
- Serve chilled and garnish with additional lemon zest, if desired.
