Set aside one teaspoon of orange juice in a small bowl.
In a small saucepan on medium heat, melt butter.
Whisk in remaining orange juice, sugar, and Grand Marnier.
Bring mixture to a low boil and cook for ten minutes to reduce the liquid.
Whisk the cornstarch into the previously set aside orange juice until it yields a uniform slurry, then add to the mixture. Continue to whisk for about one minute.
Remove from heat and let cool to thicken.
If you are making the sauce for children, you may want to skip the Grand Marnier and use just orange juice.You can leave off the butter and cornstarch slurry to make an orange syrup instead of an orange sauce.