In a heatproof bowl, combine fresh lemon juice, granulated sugar, and egg yolks. Set the bowl over a pot of simmering water, whisking constantly for about 8–10 minutes until the mixture thickens and coats the back of a spoon. Once thickened, remove from heat and set aside to cool slightly.

Once the lemon curd has cooled a bit, add chunks of room temperature unsalted butter to the mixture. Stir until the butter is fully melted and incorporated, resulting in a smooth and glossy lemon curd. Allow this mixture to cool at room temperature, letting the flavors meld together beautifully.

In a saucepan, bring a mixture of water, additional lemon juice, and sugar to a simmer. Stir until the sugar is completely dissolved, then mix in the Limoncello. Once combined, remove from heat and refrigerate the syrup until it's cool, allowing it to absorb the invigorating lemon flavors.

In a cold mixing bowl, combine mascarpone cheese, heavy cream, and sifted powdered sugar. Using an electric mixer, beat the ingredients until fluffy, which should take about 3–5 minutes. Gradually incorporate the chilled Limoncello syrup, mixing gently until fully blended and creamy, yet not overwhipped.

In a shallow dish, quickly dip ladyfingers into the cooled Limoncello syrup, ensuring they're soaked but not soggy. Layer these soaked ladyfingers in the bottom of a rectangular pan, followed by a generous layer of lemon curd and then a layer of mascarpone cream. Repeat these layers until all components are used, finishing with the mascarpone cream on top.

Cover the assembled Limoncello tiramisu loosely with plastic wrap and refrigerate for at least 4 hours to allow the flavors to meld and the dessert to set properly. This chilling step is essential for achieving the perfect texture and tempering the delicious flavors.

Before serving, spread any remaining lemon curd over the top layer of mascarpone cream. For an elegant touch, whip a small amount of heavy cream and pipe it decoratively on top. Serve chilled, garnished with fresh lemon slices or a sprinkle of zest for a vibrant finish that complements the Limoncello Tiramisu.