Generously grease a 9-inch springform pan, using one tablespoon of the butter. Preheat oven to 350° F.
Crumble the cookies into rather small pieces with your fingers. Use half to line the bottom of the springform; reserve the rest.
Place ricotta, milk, whole eggs, cornstarch, vanilla or lemon extract and the rest of the butter into the container of a food processor. Process until well-mixed and smooth. (If you don't have a food processor, mix the cornstarch into a smooth paste with a little of the cold milk before adding it to the rest of the above ingredients; then beat until smooth.)
Add a pinch of salt and cream of tartar to the egg whites; then beat at high speed until soft peaks form. Add sugar, about two tablespoons at a time, until all is incorporated. Continue beating until you cannot taste or feel a single grain of sugar.
Mix together gently but thoroughly the ricotta and egg white mixtures and pour into the prepared springform pan. Sprinkle the top of the cheesecake with the rest of the cookie crumbs. Place the pan into a larger baking dish containing about one inch of water. Bake for approximately 45 minutes, or until cake is set. (If the cookie crumbs brown too quickly, cover loosely with a piece of foil.)
Cool cake in pan until lukewarm before turning out onto a serving dish. Chill thoroughly. Garnish the top with fruit, if desired.Makes 4 Source: