Whisk yolks and eggs in medium bowl to blend. Set aside.
Combine lemon juice, butter, and sugar in large metal bowl. Set bowl over saucepan of simmering water; whisk until butter melts and sugar dissolves.
Gradually whisk 1/3 of hot butter mixture into eggs, whisk mixture back into bowl with remaining butter mixture set over simmering water. Whisk constantly until custard thickens and thermometer inserted into mixture registers 180° F, about 5 minutes (do not boil). (If you don’t have a candy thermometer, wait until the mixture thickens and coats the back of a wooden spoon.)
Pour lemon curd through strainer set over medium bowl. Place plastic wrap directly on surface of lemon curd; chill overnight.
Can be made up to 1 week ahead. Keep refrigerated.
Make lemon syrup:
Place sugar, water, and zest in a small saucepan over medium heat. Bring to a boil, let the sugar dissolve, and boil for one minute. Take off of the heat, and add in the lemon juice. The syrup will keep in the fridge for a few weeks (if it lasts that long).
Assemble lemon tiramisu:
With a stand mixer or hand-held mixer, whip cream with ½ cup powdered sugar, and ½ teaspoon lemon zest. Set aside.
In a clean large bowl, mix together mascarpone, ½ cup of powdered sugar, lemon curd, limoncello, ¼ cup lemon syrup, and 1 ½ tablespoons of lemon zest until fluffy. Fold in half the whipped cream.
In another small bowl, mix remaining lemon syrup with lemon juice.
Dollop about ½ cup of the mascarpone mixture in the bottom of a serving dish.
Using a pastry brush, brush both sides of each ladyfinger with lemon juice mixture, and lay in a single layer on top of the dolloped mascarpone.
Dollop in some of the mascarpone mixture, smooth it out, then top with another layer of lemon brushed ladyfingers. Continue layering mascarpone and lemon ladyfingers until you run out. You should end up with 2-3 layers, depending on the size of your pan.
Refrigerate at least 6 hours before serving. Garnish with lemon slices and additional lemon zest. Serve with dollops of whipped cream.
Mix any leftover syrup with club soda or vodka. Or, make lemonade with water, syrup, and lemon juice. This recipe will make more lemon curd than you need for the tiramisu. Package it in a pretty jar as a gift, or keep it for yourself to eat with biscuits, scones, or crackers.Makes 8-12 servings