Preheat oven to 350° F. Grease an 8 x 8-inch baking pan.
Place the biscotti in a food processor and grind to fine crumbs. Add the melted butter and 1 tablespoon lemon zest. Process until the crumbs begin to stick together. Press the crumbs onto the bottom of the prepared pan. Bake 15 minutes until the crust is golden brown. Remove the pan to a cooling rack and cool 10-15 minutes.
Using a clean food processor bowl, process the ricotta until smooth. Add the cream cheese, sugar, Limoncello, vanilla, and remaining lemon zest. Blend well, stopping occasionally to scrap down the sides of bowl. Add the eggs and blend until incorporated. Pour the cheese mixture over the crust.
Place the baking pan in a large roasting pan. Pour hot water in the roasting pan to come halfway up side of smaller pan. Bake approximately 1 hour. The cake should be golden with the center moving slightly when shaken. Transfer the cake pan to a wire rack and cool 1 hour. Refrigerate at least 8 hours or overnight before serving. Cut the cake into 16 squares to serve.