Line a 9x9" square pan with two sheets of parchment paper placed in a cross shape. Use binder clips to secure the paper to the rim of the pan. Set aside.
In a large mixing bowl, whisk together egg yolks and sugar.
Create a double boiler: Fill a small pot with about 1" of water. Bring it to a boil. Turn off the heat and place the mixing bowl with the egg yolk mixture on top of the pot, ensuring the bottom of the bowl doesn't touch the water.
Whisk the egg yolk mixture until most of the sugar has dissolved and the mixture becomes warm, fluffy, and pale in colour, about 5-7 minutes. Remove the bowl from the pot.
Immediately add the mascarpone cheese to the warm egg mixture. Whisk until the mascarpone cheese has completely melted into the mixture without any lumps. Set aside.
In another large mixing bowl, add whipping cream and vanilla extract. Use an electric hand mixer to beat the cream until it becomes firm peaks.
Add the whipped cream to the mascarpone mixture. Fold together with a spatula until no streaks remain. Set aside.
In a wide but shallow bowl, add the limoncello (see notes for limoncello substitute).
Dip each ladyfinger very briefly (1 second per side) into the limoncello to coat both sides and let the excess drip off. Arrange the soaked ladyfingers to cover the bottom of your lined pan.
Add half of the mascarpone cream on top of the ladyfinger layer. Use a spatula to smooth it into an even layer.
Repeat soaking ladyfingers and arranging another layer on top of the mascarpone. Top with the remaining mascarpone cream and smooth with a spatula.
Transfer your cooled lemon curd to a piping bag. Cut the tip off of the bag. Pipe an even layer of lemon curd on top of the last mascarpone layer. Use an offset spatula to very gently smooth out the lemon curd without pushing it too hard and disturbing the mascarpone cream.
Chill the limoncello tiramisu in the fridge for at least 6 hours or preferably overnight before slicing. Serve as is or garnish with dollops of whipped cream and lemon slices.