• Preheat oven to 350 degrees F and use parchment paper to line a 9-inch square baking pan, with enough to hang over the sides.

For the Crust:

• Using a microplane, finely grate 2 tablespoons of orange zest.
• Combine the flour, white and powdered sugars, and 1 tablespoon of orange zest in a large mixing bowl. Grate the extra cold butter into the flour with a cheese grater, and mix with your hands until a crumbly dough forms.
• Press the dough into the prepared pan and bake until the crust is pale golden brown, approximately 30 minutes.

For the Curd:

• While the crust is baking, begin to prepare the curd. Juice your oranges to yield 1 cup of juice.
• Whisk the orange juice, eggs, sugar, cornstarch, flour, and orange zest in a saucepan over medium heat until they slowly simmer and thicken. Once the mixture boils for 30 seconds to 1 minute, it thickens sufficiently. Remove from heat and strain into a mixing bowl. Whisk in the butter.
• Once the crust is ready, remove it from the oven and pour the orange curd onto it.
• Return the pan to the oven and continue baking; rotate after 15 minutes until the curd is set or no depression remains after touching the center; this should take around 30 minutes.
• Remove from the oven and cool to room temperature, then refrigerate until chilled through, at least 1 hour. Cut into bars and garnish with shaved chocolate (optional).