Begin by zesting and juicing 4 large lemons. Use a micro-plane for the zest. In a separate bowl, separate the yolks from 3 egg yolks and allow them to come to room temperature. Place 1 cup of heavy cream in the refrigerator to chill.

In a saucepan, combine the egg yolks, 1/3 cup of granulated sugar, and the lemon juice. Whisk vigorously over low heat for about 5-7 minutes until the mixture thickens enough to coat the back of a spoon.

In a small bowl, pour 2 tablespoons of cold water and sprinkle in 1 teaspoon of gelatin powder. Let it bloom for about 5 minutes. After blooming, dissolve it in your saucepan over low heat, stirring until fully melted, and mix into your warm lemon custard.

Retrieve your chilled heavy cream and add 1 teaspoon of vanilla extract. Whip the cream using an electric mixer until it forms soft peaks, approximately 3-5 minutes.

Fold one-third of the whipped cream into the lemon custard gently. Once incorporated, add the remaining whipped cream and continue folding until no streaks remain.

Chill your serving glasses briefly. Pipe or spoon the mousse into each glass, garnishing with fresh lemon zest. Place the filled glasses in the fridge to set for at least two hours.