Prepare the lemon curd by whisking egg yolks, sugar, lemon zest, lemon juice, and salt in a heat-proof bowl over simmering water for 7-10 minutes until thickened. Stir in butter until smooth.
Cool the lemon curd by pouring it into a bowl and covering with plastic wrap. Refrigerate while preparing other components.
Make the lemon syrup by boiling lemon juice, water, and sugar in a saucepan for about 5 minutes. Cool to room temperature.
Whip the mascarpone by combining it with lemon zest and folding in heavy whipping cream until medium-stiff peaks form.
Assemble the tiramisu by dipping ladyfingers in lemon syrup and layering them in a dish. Spread half of the mascarpone mixture and then the lemon curd. Repeat layers.
Chill the assembled tiramisu in the refrigerator for at least two hours or overnight before serving.