1Heat oven to 350°F. Spray bottom of 8-inch square pan with cooking spray.
2In large bowl, cut butter into cookie mix, using fork or pastry blender. Stir in egg until mixture is crumbly. Press half of cookie mixture into bottom of pan; bake 15 minutes.
3In small bowl, stir condensed milk, lemon peel and lemon juice until thick. Spread over baked crust. Crumble remaining cookie mixture over top. Bake about 25 minutes longer or until light golden brown. Refrigerate 30 minutes or until set. For bars, cut into 5 rows by 5 rows. Store covered in refrigerator.