1. Combine lemon juice, butter, sugar, salt, whole egg, egg yolks, and lemon zest in a saucepan. Whisk constantly over medium-low heat to prevent scrambling.

2. Continue whisking for about 5 minutes until the curd thickens to a custard-like consistency that coats the back of a spoon. Don’t increase the heat.

3. Pour the curd through a fine-mesh strainer into a bowl to remove lumps. Cover tightly with plastic wrap pressed directly onto the surface. Refrigerate for 20-30 minutes.

4. In a large bowl, combine cold heavy cream, vanilla extract, lemon zest, lemon curd, lemon juice, and sugar. Beat with an electric mixer until medium peaks form.

5. Add room temperature mascarpone cheese to the whipped cream mixture. Blend on low speed just until smooth and well combined. Don’t overmix.

6. Mix lemon syrup with Limoncello if using. Quickly dip each ladyfinger into the mixture just to soak lightly. Arrange the first layer in a 10.5 x 7.5 inch baking dish.

7. Spread half the mascarpone cream over the soaked ladyfingers. Smooth with an offset spatula. Spoon 4 tablespoons of lemon curd on top and spread gently.

8. Repeat with another layer of soaked ladyfingers and remaining mascarpone cream. Cover tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

9. Before serving, spread the remaining lemon curd on top and smooth with a spatula for that beautiful bright yellow finish.