Step 1: Make the Lemon Curd - In a medium saucepan over low heat, cream together unsalted butter, granulated sugar, and lemon zest. Gradually add in whole eggs and egg yolks, whisking continuously. Pour in lemon juice and cook, stirring constantly for about 8 minutes until thickened. Cool slightly and refrigerate until set.

Step 2: Prepare the Lemon Filling - In a heatproof bowl, combine whole eggs, egg yolks, granulated sugar, lemon zest, lemon juice, and a dash of salt. Place the bowl over simmering water and whisk constantly for about 12 minutes until thickened to custard-like consistency. Cool and refrigerate.

Step 3: Whip Egg Whites - Beat egg whites with a pinch of salt until soft peaks form. Gradually add granulated sugar and continue to beat until stiff peaks form.

Step 4: Combine Mixtures - Fold whipped egg whites into lemon filling gently. Fold in chilled lemon curd, transfer to serving dishes or bowl, and refrigerate until fully set, about 2-3 hours.

Step 5: Make Whipped Cream - Pour cold heavy whipping cream into a bowl, add powdered sugar and vanilla extract. Whip until soft peaks form. Dollop on top of set mousse and garnish as desired.