While the tart crust is baking, prepare the orange filling by first washing and drying the oranges.
Zest 3 of the oranges.
Place the zest in a medium sized bowl and add the cornstarch. Toss together until the zest is fully coated.
Juice 2 of the zested oranges, and add the orange juice to the orange zest and cornstarch mixture. Mix together with a fork.
In a separate bowl, lightly beat the eggs, and add the powdered sugar. Continue to beat the egg and sugar mixture with a whisk until it becomes light yellow in color.
Add the softened butter to the egg and sugar mixture, and mix until fully incorporated.
Add the egg mixture to the bowl with zest and orange juice. Mix until incorporated.
Pour the orange filling into the partially baked tart crust.
Place the tart in preheated oven and bake for 25 minutes.
The tart is ready when the top of the tart is golden brown in color.
Place the tart on a cooling rack, and allow it to cool for about 5 minutes before removing from the tin. Allow it to cool completely on the cooling rack prior to serving. It can be served at room temperature or chilled.