Combine cream cheese, sugar and vanilla, mixing at medium speed on electric mixer until well blended. Add eggs, one at a time, mixing well after each addition. Pour over brownie layer. Spoon chocolate over cream cheese mixture, cut through mixture several times with a knife to achieve a marble effect. Bake at 350 degrees for 35 minutes. Loosen cake from rim of pan; cool before removing rim. Chill. Garnish with whipped cream, if desired.