Make the lemon curd: Whisk the egg yolks, sugar, lemon zest, lemon juice, and salt in a heatproof bowl over a pot of simmering water. Whisk constantly for about 10 minutes, until thickened. Stir in the butter until smooth. Cover with plastic wrap pressed directly against the surface and chill.
Make the lemon syrup: Combine the water, lemon juice, and sugar in a saucepan. Bring to a boil and simmer for 5 minutes, until the sugar dissolves. Let cool completely.
Make the whipped mascarpone: Beat the mascarpone, heavy cream, and lemon zest together until stiff peaks form. Be careful not to overmix.
Assemble: Quickly dip the ladyfingers into the cooled lemon syrup and arrange them in the bottom of an 8×8-inch pan.
Spread half of the whipped mascarpone over the ladyfingers, followed by half of the lemon curd.
Repeat with another layer of dipped ladyfingers, the remaining mascarpone, and the remaining lemon curd.
Top with grated white chocolate or extra lemon zest, if desired. Cover and refrigerate for at least 4 hours before serving.
