In a medium saucepan over medium-high heat, combine all the syrup ingredients. Stir occasionally and bring the mixture to a gentle simmer. Let it simmer for about 5 minutes, or until slightly reduced and the sugar has fully dissolved. Remove the pan from heat, pour the syrup into a shallow bowl, and place it in the refrigerator to cool completely.

Whisk the egg yolks, whole egg, and sugar together in a medium saucepan until just blended. Add in the lemon juice, lemon zest, and salt, then add the butter. Cook the mixture over medium-low heat, stirring constantly, until it thickens, about 5 to 8 minutes. Once thickened, immediately strain the curd through a fine-mesh sieve into a medium bowl. Cover the surface with plastic wrap and refrigerate until fully chilled.

Using a stand mixer with the whisk attachment, whip the heavy cream until stiff peaks form. Transfer the whipped cream to a separate bowl. Swap the whisk for the paddle attachment, and in the same mixer bowl, combine the mascarpone cheese, granulated sugar, lemon syrup, and vanilla extract. Beat on high speed until smooth. Add about one cup of the whipped cream into the mixture and mix until fully incorporated. Then gently fold in the remaining whipped cream by hand to maintain the light texture.

Working one at a time, briefly dip each ladyfinger into the cooled lemon syrup, ensuring they’re soaked but still firm. Line the bottom of an 8-inch square dish with the dipped ladyfingers, arranging them in two straight rows of six. Spread half of the lemon curd evenly over the ladyfingers, followed by half of the mascarpone filling. Add a second layer of lemon syrup-dipped ladyfingers on top. Then, spread the remaining lemon curd over the second layer of ladyfingers. Follow with half of the remaining mascarpone filling, smoothing it evenly. Transfer the rest of the mascarpone filling into a piping bag fitted with a large round tip (or a zip-top bag with the corner snipped off), and pipe it decoratively across the top of the tiramisu. Cover the dish and refrigerate for a minimum of 6 hours before serving to allow the layers to set and the flavors to meld.