In a medium saucepan, cream together 1/2 cup of room temperature unsalted butter and 1 cup of granulated sugar until light and fluffy. Gradually add in the zest of 2 lemons and a pinch of salt. Incorporate 3 whole eggs one at a time, whisking well after each addition. Stir in 1 cup of freshly squeezed lemon juice and cook over medium heat for about 8 minutes until the mixture thickens. Remove from heat and cool, then store the lemon curd in the refrigerator.

In a heatproof bowl set over simmering water, combine 3 eggs, 3 egg yolks, 1/3 cup of sugar, 2 tablespoons of lemon zest, 1/2 cup of lemon juice, and a pinch of salt. Whisk continuously for approximately 12 minutes, until the mixture thickens and coats the back of a spoon. Once thickened, remove it from the heat and let it cool before transferring it to the refrigerator for at least 30 minutes.

In a clean mixing bowl, whip 3 egg whites with a pinch of salt using an electric mixer until soft peaks form. Gradually add in 1/4 cup of granulated sugar while continuing to beat until you achieve stiff peaks. The egg whites should turn glossy and hold their shape firmly, which adds lightness to your lemon mousse.

Gently fold the cooled lemon filling into the beaten egg whites, taking care not to deflate the mixture. Once fully combined, add in the chilled lemon curd, folding until the batter is smooth and well combined. Transfer this airy lemon mousse mixture into a souffle dish or individual serving cups. Chill in the refrigerator for at least 2 hours or until set.

In a mixing bowl, whip 1 cup of heavy whipping cream along with 2 tablespoons of powdered sugar and 1 teaspoon of pure vanilla extract until soft peaks form. The whipped cream should be fluffy and hold well without being overly stiff. Use this lovely cream to garnish your set lemon mousse, adding a delicious creamy contrast to the zesty dessert.