Heat oven to 350 degrees. Spray a 9x13 inch pan with cooking spray.

Place cookie mix in a large bowl. With a pastry blender or fork, cut in butter until mixture is crumbly. Reserve 3 cups of the crumb mixture. Press the remaining crumb mixture into the bottom of the prepared pan. Bake 10 minutes.

In small bowl, stir together sugar and cornstarch. Gently stir in yogurt and vanilla. Spoon yogurt mixture evenly over crust. Sprinkle blackberries evenly over yogurt. Top evenly with reserved crumb mixture.

Bake for about 45 minutes or until light golden brown. Remove to a wire rack and cool for 30 minutes. Refrigerate for at least 2 hours or until thoroughly chilled before cutting into bars. Store leftover bars, covered in the refrigerator.