Preheat the oven to 350 degrees F. Line a 13x9" baking pan with foil and spray the foil with cooking spray; set aside.

In a large bowl, combine the flour and sugar. Cut in the cubed butter with a pastry blender or two forks until the mixture forms coarse, moist crumbs. You'll know it's ready when you grab a fistful and it holds together. Spread the mixture into an even layer into the prepared baking pan. Bake for approx. 7 minutes. Remove from oven and keep oven on.

While crust bakes, make your filling. In a large bowl, whisk together the sugar and flour until no clumps remain. Add in the eggs, orange juice and orange zest and whisk vigorously to combine, until smooth. Pour the filling mixture over the crust and bake for approx. 20 minutes or until set. Cool completely, then refrigerate for about 2 hours or until set. Dust with powdered sugar before serving.