Preheat your oven to 350°F (175°C) and grease an 8x8 or 9x13 inch baking pan.

In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and orange zest.

In a separate bowl, mix melted butter with eggs until well combined. Incorporate wet mixture into dry ingredients.

Pour batter into the prepared baking pan and spread evenly.

In another bowl, combine sugar, cornstarch, orange juice, zest, eggs, and vanilla. Whisk until smooth.

Pour filling over batter and bake for 25-35 minutes for a 9x13 pan, or 35-45 minutes for an 8x8 pan.

Cool completely at room temperature before glazing.

Mix powdered sugar with orange juice for glaze, drizzle over bars, and cut into squares.