Step 1: Prepare the Lemon Syrup

1. In a small saucepan, combine the lemon juice, water, and granulated sugar.
2. Heat over medium-low heat until the sugar dissolves completely, stirring occasionally.
3. Remove from heat and stir in the lemon zest and limoncello (if using).
4. Allow the syrup to cool completely to room temperature.

Step 2: Prepare the Mascarpone Filling

Beat the Egg Yolks

1. In a large bowl, beat the egg yolks with half of the sugar (about 6 tablespoons) until pale and creamy.
2. Add the mascarpone cheese and beat until smooth and well combined.
3. Fold in the lemon zest and vanilla extract.

Whip the Egg Whites

1. In a separate clean bowl, beat the egg whites until foamy.
2. Gradually add the remaining sugar and continue beating until stiff peaks form.
3. Gently fold the egg whites into the mascarpone mixture using a rubber spatula.

Whip the Cream

1. In another bowl, whip the heavy cream until stiff peaks form.
2. Gently fold the whipped cream into the mascarpone mixture until smooth and fully incorporated.

Step 3: Assemble the Lemon Tiramisu

1. Pour the lemon syrup into a shallow dish.
2. Quickly dip each ladyfinger into the lemon syrup (about 2-3 seconds per side) – don’t soak them too long or they’ll become soggy.
3. Arrange a layer of soaked ladyfingers in the bottom of an 8×8 inch glass dish or a 9-inch springform pan.
4. Spread half of the mascarpone filling evenly over the ladyfingers.
5. Create a second layer of dipped ladyfingers.
6. Top with the remaining mascarpone filling and smooth the surface.

Step 4: Chill and Serve

1. Cover the Lemon Tiramisu with plastic wrap and refrigerate for at least 4 hours, preferably overnight.
2. Before serving, dust the top with powdered sugar.
3. Garnish with thin lemon slices or additional lemon zest if desired.