In a medium saucepan, combine 2/3 cup granulated sugar, 1/3 cup butter, 1/4 teaspoon kosher salt, and 4 tablespoons lemon juice. Over medium heat, stir until the butter melts and the mixture is smooth. Whisk in 1 egg and 3 egg yolks, cooking while stirring constantly for about 8-10 minutes until thickened. Remove from heat and allow to cool.

In a large mixing bowl, pour in 2 cups of heavy cream that has been chilled. Beat the cream on medium speed until soft peaks form, about 2-3 minutes. Gradually add in 2/3 cup granulated sugar, continuing to beat until stiff peaks form. Gently fold in 15.87 oz mascarpone cheese, 4 tablespoons lemon juice, 2 tablespoons lemon zest, and the cooled lemon curd until fully blended.

In a separate dish, mix together 1 cup lemon syrup, 1/4 cup limoncello, and 1/3 cup lemon juice. Quickly dip each ladyfinger into the mixture for no more than 2 seconds on each side. Set them aside on a clean plate until ready to layer.

In a rectangular dish, layer half of the soaked ladyfingers at the bottom. Spread half of the creamy mascarpone mixture evenly over the ladyfingers. Repeat this with another layer of soaked ladyfingers and the remaining mascarpone mixture.

Cover the assembled Lemon Tiramisu with plastic wrap and chill in the refrigerator for at least 4 hours, ideally overnight.