In a medium saucepan, cream together unsalted butter, granulated sugar, salt, and lemon zest over low heat until smooth, about 5 minutes.

Gradually add the large eggs, one at a time, whisking well after each addition. Stir in freshly squeezed lemon juice and continue cooking, stirring constantly, until thickened, about 10 minutes.

Remove from heat and allow to cool, storing in the refrigerator until needed.

In a heatproof bowl over simmering water, whisk together the eggs, egg yolks, sugar, lemon zest, lemon juice, and salt.

Cook, whisking continuously, until pudding-like consistency is reached, about 12 minutes. Remove from heat and cool completely.

In a clean stand mixer bowl, whip egg whites with a pinch of salt on medium speed.

Gradually add granulated sugar and increase speed to high until stiff peaks form, about 5-7 minutes.

Gently fold whipped egg whites into chilled lemon filling in batches, then incorporate lemon curd.

Transfer to a serving dish and cover with plastic wrap, refrigerating for at least 4 hours.

In a chilled bowl, whip heavy cream with powdered sugar and vanilla until soft peaks form, about 3-5 minutes.

Serve mousse topped with whipped cream, garnishing with fresh berries or mint leaves.