1Heat oven to 350°F. Spray bottom only of 13x9-inch pan with cooking spray.
2In large bowl, stir cookie mix and 2 tablespoons flour. Cut in butter, using pastry blender (or pulling 2 table knives through ingredients in opposite directions), until mixture looks like fine crumbs. Press mixture in bottom of pan. Bake 13 to 15 minutes or until edges are lightly browned.
3Meanwhile, in medium bowl, mix egg yolks, condensed milk, 1 tablespoon flour, the lemon peel and lemon juice with whisk. Pour over hot crust. Bake 18 to 20 minutes longer or until filling is set. Cool on cooling rack 1 hour. Cover; refrigerate until chilled, about 1 hour.
4Sprinkle bars with powdered sugar. Cut into 6 rows by 4 rows. Store covered in refrigerator.