Zest and juice lemons and set aside.

Lemon Curd: Add 2 inches of water to the bottom of a small saucepan and bring to a simmer over medium heat. In a heat-proof mixing bowl, add 4 egg yolks, 2/3 cup granulated sugar, 1 Tablespoon lemon zest, 1/3 cup lemon juice, and a pinch of salt and whisk well to combine. Place bowl over pot of water, reduce heat to medium low and cook, whisking constantly, until mixture thickens to a gravy consistency, about 7-10 minutes. Remove bowl from heat and stir in 6 tablespoons butter.

Cool Lemon Curd: Pour mixture into a new bowl and place a piece of plastic wrap directly on the surface (to prevent it from forming a skin). Refrigerate whie you prepare the rest of the ingredients.

Lemon Syrup: In a saucepan, add 3/4 cup lemon juice, 1 cup water, and 1/2 cup sugar. Bring to a boil, stirring to dissolve the sugar. Boil for 5 minutes, then remove from heat. Transfer to a bowl and cool completely to room temperature (or refrigerate to cool down faster). Stir in 1/4 cup limoncello (if using).

Whipped Mascarpone: Combine mascarpone with 1 Tablespoon lemon zest then mix in the whipping cream with an electric mixer just until medium/stiff peaks (don't over-mix).

Assemble: Submerge each lady finger quickly in lemon syrup and then place in the bottom of an 8x8 or similar size pan, until bottom of pan is lined. Top with half of the whipped mascarpone. Stir lemon curd well then spoon over the top, into an even layer. Repeat with another layer of dipped lady fingers. Smooth (or decoratively pipe) remaining whipped mascarpone on top. Optional: garnish with grated white chocolate and/or lemon zest or lemon slices.

Refrigerate for at least 2 hours, or overnight.