Preheat the oven to 350 degrees F. Spray the bottom of a 9 by 9 by 2-inch baking pan with nonstick cooking spray.
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 on a cooling rack.
Blend the ricotta in a clean food processor until smooth. Add the cream cheese and sugar and blend well, stopping the machine occasionally and scraping down the sides of the work bowl. Blend in the limoncello, vanilla, and remaining 2 tablespoons of lemon zest. Add the eggs 1 at a time, and pulse just until blended. Scrape down the sides of the bowl as needed.
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 1 hour (the cake will become firm when it is cold).
Transfer the cake to a rack; cool 1 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.
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