In a small pot or saucepan, stir together whole eggs, egg yolks, sugar, and lemon juice. Heat on medium heat, stirring constantly, until thickened, about 10-15 minutes. Remove from heat and stir in butter until melted. Push through a fine mesh sieve to remove lumps (optional) and cool completely.

Line a 9x9" square pan with parchment paper in a cross shape. Secure with binder clips.

In a large bowl, whisk egg yolks and sugar. Create a double boiler by filling a pot with 1" of water, boil, then turn off heat and place the bowl on top. Whisk until warm, fluffy, and pale, about 5-7 minutes. Remove from pot.

Immediately add mascarpone cheese to the warm egg mixture and whisk until smooth.

In another bowl, beat whipping cream and vanilla extract with an electric mixer until firm peaks form. Fold whipped cream into mascarpone mixture until no streaks remain.

In a shallow bowl, add limoncello. Briefly dip each ladyfinger into limoncello to coat both sides, letting excess drip off. Arrange soaked ladyfingers in the bottom of the lined pan.

Spread half of the mascarpone cream over the ladyfingers and smooth into an even layer. Repeat with another layer of soaked ladyfingers and remaining mascarpone cream.

Transfer lemon curd to a piping bag and pipe an even layer over the last mascarpone layer. Smooth gently with an offset spatula.

Chill in the fridge for at least 6 hours or overnight before slicing. Serve as is or garnish with whipped cream and lemon slices.