Preheat oven to 375 F. Butter 8-inch square baking pan. Mix poppy seed filling, lemon juice, lemon peel and vanilla in medium bowl. Beat egg white in small bowl until stiff but not dry. Fold white into filling. Set aside.
Mix flour, sugar, baking powder and salt in large bowl. Using fingertips, rub Olive Lea Spread into dry ingredients until mixture resembles coarse meal. Mix egg and sour cream in small bowl. Add to flour mixture; stir until moist clumps form. Gather dough together; divide in half. Press 1 dough piece onto bottom of prepared pan.
Bake crust until golden, about 15 minutes. Spread filling over crust. On floured surface, press second dough piece out to 8-inch square. Place atop filling; press to adhere. Brush with glaze.
Bake until crust is golden, about 35 minutes. Sift powdered sugar over. Cool in pan on rack. Cut pastry into squares.