Step 1

Preheat oven to 350 degrees F (175 degrees C). Line a 7 x 11-inch baking pan with foil; spray with cooking spray.

Step 2

Combine flour, 1/4 cup cornstarch, 1/4 cup powdered sugar, and white sugar in a bowl. Mix in the butter; and stir until well mixed and crumbly. Press mixture in bottom of prepared pan.

Step 3

Bake in preheated oven until lightly golden, about 15 minutes.

Step 4

Mix 3/4 cup white sugar, eggs, 2 tablespoons cornstarch, baking powder, lemon juice and lemon zest together in a mixing bowl using an electric mixer or wire whisk. Blend thoroughly.

Step 5

Pour filling over hot crust. Return pan to oven and continue baking until filling is set, 18 to 20 minutes.

Step 6

Cool completely before cutting into bars. Sprinkle with powdered sugar if desired.