** Place plain, nonfat yogurt into cheese cloth or a coffee filter. Place in strainer over a small bowl or cup. Allow to drain 2 to 24 hours, covered and refrigerated, until cheese reached desired consistency. Discard the whey (liquid). Store in covered container in refrigerator.
Preheat oven TO 350F. Spray 8-inch springform pan with cooking spray. Fill a 13x9x2-inch pan halfway with hot water.
Place in oven on shelf below where chocolate cheesecake will be placed. This will prevent the top of cheesecake from cracking.
In small bowl, combine all crust ingredients. Press onto bottom and part-way up sides of pan. Set aside.
In food processor or blender, combine egg whites, sugar, ricotta, cocoa, yogurt cheese and vanilla; process, scraping sides occasionally, for 1 minute until smooth and thick. Pour into crust.
Bake 30 to 35 minutes until filling is just set (center will jiggle slightly). Turn off oven and leave cake in closed oven 5 minutes longer.
Remove from oven and cool about an hour on wire rack. Cover and refrigerate at least 3 hours before serving.