CRUST:

1 1/2 c. all-purpose flour
1/4 c. powdered sugar
Pinch of salt
1/2 c. (1 stick) chilled unsalted butter, cut into pieces
1/2 tsp. vanilla

TOPPING:

5 lg. eggs
2 c. sugar
1 c. strained fresh lemon juice
3 tbsp. all-purpose flour
2 1/2 tbsp. grated lemon peel
Powdered sugar

FOR CRUST: Position rack in center of oven and preheat to 350 degrees. Line 9-inch square baking pan with foil, extending foil 1 inch above 2 sides of pan. Butter 2 uncovered sides of pan. Combine flour, powdered sugar and salt in processor. Add butter and cut in using on-off turns until mixture appears sandy.

Add vanilla and process until dough begins to come together. Press dough evenly into prepared baking pan. Bake crust until golden brown, about 28 minutes.

MEANWHILE, PREPARE TOPPING: Whisk eggs and 2 cups sugar in medium bowl to blend. Whisk in lemon juice, then all-purpose flour. Strain into another bowl. Mix in grated lemon peel.

Reduce oven temperature to 325 degrees. Pour filling over hot crust. Bake until sides are set and filling no longer moves in center when pan is shaken, about 22 minutes. Cool on rack. Cover and chill at least 4 hours or overnight.

Using foil sides as aid, lift dessert from pan. Fold down foil sides. Cut into 16 squares. Cut each square diagonally in half forming triangle. Sift powdered sugar over. (Can be prepared 1-2 days ahead. Refrigerate airtight).