Begin by zesting the lemons, taking care to avoid the bitter white pith; you'll need both the zest and juice for your lemon mousse. Separate the egg yolks from the whites, placing the yolks in a medium saucepan. Chill your heavy cream in the refrigerator so it’s ready to whip later, ensuring your dessert reaches that light, airy texture.

In the saucepan with the egg yolks, add granulated sugar and whisk vigorously until smooth and creamy. Then, slowly add the freshly squeezed lemon juice, combining until well blended. Place the pan over medium heat and cook the mixture, stirring constantly for 5-7 minutes, until it thickens and coats the back of a spoon, achieving a custard-like consistency.

While the custard cools, bloom the gelatin by sprinkling it over 2 tablespoons of cold water in a small bowl. Let it sit for about 5 minutes until it softens. After this, gently heat the bloomed gelatin in the microwave for about 10-15 seconds until fully dissolved. Stir the melted gelatin into the cooled lemon custard, ensuring even distribution.

Next, remove your chilled heavy cream from the refrigerator and pour it into a mixing bowl. Add the vanilla extract, then use a hand mixer or a whisk to beat the cream until soft peaks form, taking care not to over-whip. This whipped cream will give your lemon mousse its signature lightness, so keep an eye on it as you whip.

Now, it’s time to bring everything together. Start by folding one-third of the whipped cream into the lemon custard; this will lighten the mixture. Use a rubber spatula to gently fold, ensuring not to deflate the airy cream. Once combined, carefully fold in the remaining whipped cream until fully incorporated, preserving that fluffy texture essential to your lemon mousse.

Once your lemon mousse mixture is ready, spoon or pipe it into individual serving glasses or bowls. You can use a piping bag for a more elegant presentation if desired. Finely zest a lemon on top of each serving for garnish and then place them in the refrigerator to chill for at least 2 hours; this allows the flavors to meld and the mousse to set perfectly.