In a medium bowl or your stand mixer with the paddle attachment, cream together 1/2 cup sugar and 1 cup butter until well blended. Add the 2 cups all purpose flour, 1/4 teaspoon salt, and 1 teaspoon orange zest and blend just until a crumb mixture forms. Press evenly into a 13x9 inch pan. Bake for 20-25 minutes or until lightly browned. Remove from oven and allow to cool.
While crust is cooking, beat together the 4 large eggs, 1 1/2 cups sugar, 2/3 cup orange juice, and 1/2 teaspoon orange zest. Sift the 1/4 cup flour and 1/4 teaspoon baking powder together and add to the orange mixture. Mix until combined. Pour over the cooled crust.
Bake for an additional 25 minutes.
Allow to fully cool and sprinkle with powdered sugar. Cut into bars.
Notes
Refrigerate in an airtight container for up to 5 days.
Freeze for longer storage - Wrap individual bars tightly and freeze for up to 2 months.