In a medium saucepan, combine egg yolks, whole eggs, salt, sugar, lemon zest, and lemon juice. Whisk to combine.

Heat on medium heat, whisking or stirring constantly. Let the lemon curd come to a simmer and thicken - approximately one minute after simmering.

Once thickened, strain the curd through a wire strainer or cheesecloth to remove the zest and any lumps. Transfer to a non-metal heat safe bowl. Add butter to the curd and let sit for a minute to melt. Stir until fully melted and smooth.

Cover with parchment paper and refrigerate until cold and thickened.

While the lemon curd chills, prepare the lemon syrup. Combine water, sugar, lemon juice, peeled lemon zest, and basil (optional).

Bring to a simmer on medium heat and let cool. Once cooled, strain out the lemon peels and set aside in a bowl for later. If using limoncello, add it in at this point.

Chill your mixing bowl and whisk in the freezer for 5 minutes. Pour heavy whipping cream into the chilled mixing bowl and whip to stiff peaks.

In a large bowl, combine half of the cooled lemon curd and all of the mascarpone cheese. Whisk until fully combined and smooth.

Add whipped cream into the lemon curd mixture. Gently fold to combine, being careful not to deflate the whipped cream.

Dip ladyfingers into the lemon syrup one at a time, placing them in tight rows inside the baking dish.

Add half the filling on top, spreading all the way to the edges. Place another layer of dipped ladyfingers on top.

Add the rest of the filling on top of the second layer of ladyfingers. Cover and refrigerate for 8 hours or overnight.

Before serving, top with the remaining lemon curd and serve with lemon slices.