In a medium saucepan over medium heat, combine the lemon zest and granulated sugar.
Beat the egg yolks into the sugar mixture until smooth. Add the whipping cream and continue to stir over gentle heat until the mixture thickens, about 5 minutes.
Remove from heat and stir in the vanilla extract. Let the mixture cool to room temperature.
In a large bowl, whip the heavy cream until stiff peaks form. Gently fold the cream into the lemon mixture until well combined.
Spoon the mousse into serving glasses. Cover and refrigerate for at least 2 hours or until set.
Serve chilled with optional garnish like mint leaves or a lemon twist.
Notes
For an extra zing, consider adding a tablespoon of lemon juice to the mixture.