- Make the filling: Preheat oven to 425 degrees. In a medium saucepan over medium heat, combine sugar, cornstarch, salt, and orange juice. Stir until clear and thickened, about 4 to 5 minutes. Set aside.
- In a mixing bowl, lightly beat egg yolks. Stir in half the hot sugar mixture, then transfer egg mixture to saucepan. Continue cooking over medium heat, stirring constantly, for 2 to 3 minutes. Remove from heat and stir in lemon juice, orange zest, and butter until butter melts. Pour mixture into pie shell.
- Make the meringue: In a stand mixer fitted with whisk attachment, beat egg whites and cream of tartar on medium speed until frothy. Gradually add granulated sugar and powdered sugar. Once combined, increase mixer speed to high and beat until soft peaks form, 3 to 5 minutes. Swirl meringue over pie filling.
- Bake pie on center rack until top of meringue begins to brown, 8 to 10 minutes.
From But First, Dessert.