Heat oven to 350°F. In medium bowl, mix crust ingredients. Press in bottom of ungreased 13x9inch pan. Freeze crust while preparing filling.
In large bowl with electric mixer, beat all filling ingredients on low speed until smooth. Pour into crustlined pan. Bake 30 to 35 minutes or until knife inserted in center comes out clean. Cool on wire rack.
Meanwhile, in 1-quart saucepan, melt chocolate over low heat, stirring constantly. Cool 5 minutes; beat in sour cream with spoon. Spread over warm cheesecake. Refrigerate 3 hours or until firm. Cut into squares. Store in refrigerator.