Combine all of the ingredients in a medium saucepan over medium-high heat. Bring to a simmer, stirring occasionally to dissolve the sugar. Allow mixture to simmer for 5 minutes, or until slightly reduced. Remove from heat, and pour into a shallow bowl. Place in refrigerator to cool completely.

In a medium saucepan, whisk the egg yolks, whole egg, and sugar together until just combined. Whisk in the lemon juice, zest, and salt. Add the butter and cook over medium-low heat, stirring constantly, until the mixture has thickened, about 5-8 minutes minutes.

Immediately strain the curd through a fine-mesh strainer into a medium-size bowl. Cover with plastic wrap and place in the refrigerator to cool completely.

In a stand mixer fitted with the whisk attachment, whip the heavy cream until stiff peaks form. Transfer whipped cream to a separate bowl, return bowl to the mixer, and replace whisk attachment with paddle attachment.

To the mixer, add the mascarpone cheese, sugar, lemon syrup, and vanilla. Beat on high until combined. Add about a cup of the whipped cream and continue beating until incorporated. Gently fold in the rest of the whipped cream.

One at a time, quickly dip the ladyfingers in the lemon syrup until soaked but not soggy; arrange in an even layer along the bottom of an 8-inch square dish. (Line the ladyfingers up in two straight rows of six.**) Spread half of the the lemon curd on top of the ladyfingers, followed by half of the mascarpone filling. Place another layer of lemon syrup-soaked ladyfingers on top of the filling.

Spread the rest of the lemon curd on top of the ladyfingers. Then spread half of the remaining mascarpone filling on top. Transfer remaining filling to a pastry bag fitted with a large round tip (or a gallon ziplock bag with the corner cut off), and pipe the filling evenly over the tiramisu.

Cover and refrigerate for at least 6 hours before serving.