Add yogurt cheese, egg whites, sugar, cocoa, flour and vanilla. Process just until well blended.
Pour over crumbs.
Bake 50 minutes or until edges are set.
Turn off oven. Leave in oven with door ajar for 1 hour.
Remove from oven.
Refrigerate until thoroughly chilled.
YOGURT CHEESE: line a non rusting colander or sieve with double thickness cheesecloth. Place yogurt in colander over a deep bowl. Refrigerate until liquid no longer drains from yogurt -- about 24 hours. Discard liquid.