An intense lemon-lime filling is cradled by a shortbread-style butter crust, which is made especially tender by using a combination of all-purpose and cake flours. Prep time does not include time needed to chill the crust. By Elinor Klivans
Spray 10-inch tart pan with removable bottom with cooking spray.
Whisk all-purpose flour, cake flour and salt in medium bowl.
Beat butter and powdered sugar in large bowl at medium speed until smooth. Beat in 1 egg yolk and vanilla until combined. Stir in flour mixture just until combined. Shape into flat round; cover and refrigerate 30 to 60 minutes or until firm.
Roll dough between two sheets of waxed paper into 12-inch round 1/8 inch thick. Remove top piece of waxed paper; invert dough into tart pan. Remove waxed paper; carefully press dough into pan. Trim dough even with top of pan; freeze 30 minutes.
Meanwhile, heat oven to 375°F.
Line crust with foil, covering edges; fill with pie weights or dried beans. Bake 15 minutes; remove foil and weights. Reduce oven temperature to 350°F Bake 10 minutes to firm and lightly brown crust. Cool on wire rack while preparing filling.
Whisk condensed milk and lemon and lime juices in large bowl until blended. Whisk in 5 egg yolks and lemon peel until blended; pour into slightly cooled crust.
Bake at 350°F for 30 minutes or until center is firm and set. Cool on wire rack to room temperature, about 1 hour. Cover and refrigerate 8 hours or overnight. Serve chilled. Refrigerate leftovers.