Lemon Tiramisu

Steps

1. Lemon curd: zest 2 lemons and set the zest aside. Juice 3 lemons to get 100 ml (scant 1/2 cup) juice. In a heavy saucepan, off the heat, whisk the eggs, 100 g (1/2 cup) of the sugar and the pinch of salt until smooth, then whisk in the lemon juice.
2. Set the pan over low heat and whisk constantly, scraping the sides and bottom, for 6 to 8 minutes. The mixture foams at first, then the foam subsides and it suddenly thickens: it is ready at 176°F (80°C), when it coats a spoon and a finger drawn across leaves a clean line. Don't let it boil, or the eggs will curdle.
3. Take it off the heat and whisk in the butter in small cubes, one or two at a time, until smooth and glossy. Strain it through a fine sieve into a bowl, stir in the reserved zest, press plastic wrap onto the surface and chill completely, about 1 hour (20 minutes set in an ice bath). You'll have about 14 oz (390 g) of curd.
4. Syrup: in a small saucepan, bring the water and the remaining 50 g (1/4 cup) sugar to a simmer and cook 1 minute, until the sugar dissolves. Off the heat, let it cool slightly, then add the juice of 2 lemons (about 1/4 cup) and the limoncello. Pour into a shallow dish and let cool.
5. Cream: in a large bowl, whisk the cold mascarpone with the powdered sugar and 150 g (about 1/2 cup) of the cold lemon curd just until smooth, about 30 seconds. Don't whisk any longer: overworked mascarpone turns grainy.
6. In another chilled bowl, whip the heavy cream, straight from the fridge, until it holds soft peaks that curl over at the tip of the whisk. Fold it into the mascarpone in two additions with a spatula, lifting from the bottom to keep the air in.
7. Dip the ladyfingers one at a time into the lemon syrup, 1 second per side, no more: they should be moist on the outside and still firm in the center. Line them up side by side in an 8 x 11-inch (20 x 28 cm) dish to cover the bottom.
8. Spread half the mascarpone cream evenly over the ladyfingers. Dot 120 g (about 1/3 cup) of the lemon curd over the cream in small spoonfuls and swirl it with the tip of a knife in loops, without scraping down to the cookies.
9. Dip the rest of the ladyfingers the same way and make a second layer. Cover with the remaining cream and smooth the top with a spatula.
10. Dot the remaining lemon curd (about 1/3 cup) over the top and swirl it the same way. Cover the dish and refrigerate at least 6 hours, ideally overnight.
11. Just before serving, grate the zest of the last lemon over the tiramisu. Cut portions with a knife dipped in hot water and wiped between cuts for clean edges.