2) Finely grind the biscotti in a food processor. Add the melted butter and 1 tablespoon of lemon zest, and process until the crumbs are moistened. Press the crumb mixture over the bottom (not the sides) of the prepared pan. Bake until the crust is golden, about 15 minutes. Cool the crust completely.
3) Blend the ricotta in a clean food processor until smooth. Add the cream cheese and caster sugar and blend well, stopping the machine occasionally and scraping down the sides of the work bowl. Blend in the limoncello, vanilla, and remaining two tablespoons of lemon zest. Add the eggs one at a time, and pulse just until blended. Scrape down the sides of the bowl as needed.
4) Pour the cheese mixture over the crust in the pan. Place the baking pan in a large roasting pan. Pour enough hot water into the roasting pan to come halfway up the sides of the baking pan. Bake until the cheesecake is golden and the center of the cake moves slightly when the pan is gently shaken, about one hour (the cake will become firm when it is cold).
5) Transfer the cake to a rack; cool for one hour. Refrigerate until the cheesecake is cold, at least 8 hours and up to 2 days. Cut the cake into squares and serve.
*Limoncello liqueur can be found at most liquor stores and in the liquor department of some specialty markets. If limoncello liqueur is unavailable at stores near you, use the following recipe to make your own limoncello liqueur.
Using a vegetable peeler, remove the peel from the lemons in long strips (reserve the lemons for another use). Using a small sharp knife, trim away the white pith from the lemon peels; discard the pith. Place the lemon peels in a 4L pitcher. Pour the vodka over the peels and cover with plastic wrap. Steep the lemon peels in the vodka for 4 days at room temperature.
Stir the water and caster sugar in a large saucepan over medium heat until the caster sugar dissolves, about 5 minutes. cool completely. Pour the caster sugar syrup over the vodka mixture. Cover and let stand at room temperature overnight.
Strain the limoncello through a mesh strainer. Discard the peels. Transfer the limoncello to bottles. Seal the bottles and refrigerate until cold, at least 4 hours and up to 1 month.