In a double boiler over medium-high heat, combine lemon juice, sugar, and water, stirring until the sugar dissolves, about 3-5 minutes. Whisk together the eggs and cornstarch in a separate bowl. Gradually pour the egg mixture into the double boiler, whisking constantly for 6-9 minutes until thickened and glossy. Incorporate butter if desired, then cover and chill in the refrigerator until set.

In a large mixing bowl, combine the mascarpone cream, whipping cream, and powdered sugar. Using an electric mixer on high speed, whip for about 1-2 minutes until light and fluffy, with soft peaks. Refrigerate the filling for at least 15 minutes to stabilize.

Pour warm milk into a shallow dish. Briefly immerse each ladyfinger in the warm milk, soaking for 2-3 seconds on each side. Arrange the soaked ladyfingers in a deep dish.

Layer the tiramisu by spreading lemon curd over the soaked ladyfingers, followed by a quarter of the mascarpone filling. Repeat until all components are used, ending with the mascarpone mixture.

Top the tiramisu with lemon zest and mint leaves. Cover and refrigerate for at least 4 hours, or overnight to set. Serve chilled.