In a large mixing bowl, cream together room-temperature unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes. Gradually add in large eggs one at a time, blending well after each addition. Stir in freshly squeezed lemon juice and zest, then pour the mixture into a saucepan. Cook on low heat for approximately 8 minutes, stirring continuously until thickened. Once thick, cool to room temperature and then refrigerate to set.

Using a heatproof bowl, combine eggs, egg yolks, granulated sugar, lemon zest, lemon juice, and a pinch of salt. Place the bowl over a simmering pot of water, whisking continuously for about 12 minutes or until the mixture thickens and coats the back of a spoon. Once thickened, remove the bowl from heat, let it cool, and transfer it to the fridge until ready to use.

In a clean mixing bowl, beat egg whites with a pinch of salt until they reach soft peaks, about 3-4 minutes. Gradually add in sugar and continue to whip until stiff peaks form. Set aside.

Gently fold the whipped egg whites into the cooled lemon filling, ensuring not to deflate. Add the chilled lemon curd and carefully fold until no visible streaks remain. Transfer into serving dishes or ramekins.

Cover the serving dish with plastic wrap or a lid and refrigerate the mousse for at least two hours.

In a separate chilled mixing bowl, add heavy whipping cream, powdered sugar, and vanilla extract. Whip until fluffy and holds soft peaks.

Once the mousse is set, remove from the refrigerator and dollop whipped cream on top. Garnish with lemon slices or fresh mint.