Yogurt Cheese:Line a non-rusting colander or sieve with large piece of double thick cheesecloth;place colander over deep bowl.Spoon yogurt into prepared colander;cover refrigerate until liquid no longer drains from yogurt,about 24hours. Remove yogurt from cheesecloth,discard liquid.
Heat oven to 325*.Line 9in springform pan with crushed graham crackers.
In food processor bowl blend ricotta cheese till smooth.add yogurt cheese,egg whites,sugar,cocoa, flour and vanilla.Beat just until blended.Pour over crumbs.
Bake 50minutes or until edges are set.Turn off oven;open door slightly,leave cheesecake in oven 1hour.Remove from oven.
With knife,loosencake from sides of pan..Cool completely,remove sides of pan.Refrigerate before serving.Garnish with sliced fruit.