Heat oven to 350 degrees F (175 degrees C). Line a 9x12-inch baking dish with foil (to make lifting out easy). Lightly spray the foil with Non-Stick Baking Spray (to taste).
In a large bowl, stir together All-Purpose Flour (1 1/2 cup) and Powdered Confectioners Sugar (1/2 cup). Cut the cold Butter (3/4 cup) into small pieces.
Add to the flour mixture and using a pastry cutter or the back of a fork, cut the butter until it is in pea-sized pieces. Halfway through, add 1/4 tsp zest of the Sunkist Orange (1) and 1/4 tsp zest of the Lemon (1)
Press the crust mix into the bottom of the prepared pan. Bake for 15 minutes. Remove and set aside.
While the crust is baking, in the same bowl, whip Egg (3) on high for about 3 minutes until light and frothy.
Reduce speed to low, and beat in the Granulated Sugar (1 cup), remaining All-Purpose Flour (3 tablespoon), Baking Powder (1/2 teaspoon), Salt (1/2 teaspoon), and 1/3 cup of combined orange and lemon juice.
When combined, pour over the prepared crust. Return to the oven and bake for 15 to 20 minutes until the filling is set.
Remove from the oven and carefully lift the foil and transfer to a wire cooling rack. Let cool completely, sprinkle with powdered sugar if desired, cut and serve. Enjoy!