In this luscious bar cookie, the Key lime filling is sandwiched between a rich crust and a pistachio crumb topping.
Heat oven to 350 degrees F. Line 13 x 9 x 2-inch baking pan with nonstick foil or regular foil coated with nonstick cooking spray.
Set aside 3/4 cup cake mix. In large bowl, combine remaining cake mix, egg white, butter, rind and juice. Beat with hand mixer until coarse crumbs form. Press over bottom of prepared pan. Prick with fork.
Bake in 350 degree oven 15 minutes.
In small bowl, beat 4 yolks until light colored and slightly thickened, 3 minutes. Stir in milk. Beat in juice in two parts. Stir in rind.
In small bowl, combine the reserved 3/4 cup cake mix from the crust above and the nuts. With hands, work butter into cake mix mixture until coarse crumbs form.
Remove crust from oven. Pour filling evenly over crust, smoothing to edges. Sprinkle evenly with crumb.
Bake in 350 degree F oven until set, 15 minutes. Let cool in pan on rack 20 minutes. Refrigerate to chill, 2 hours.
Remove from refrigerator; using ends of aluminum foil, lift bar from pan. Cut into 48 pieces and serve. (Or refrigerate, covered, for up to 1 week before serving.) Makes 48 bars.