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