.: Intro :.

Custard powder is a powdered mix used to make custard.  It contains flavorings and light yellow coloring.  According to the Wikipedia, the powder is a mix of corn flour, vanilla extract, and annatto coloring and other flavorings.  Combine it with water, it can then create a imitated custard sauce
Custard powder formula is found by Alfred Bird in 1837, because his wife was allergic to eggs. 

.: Characteristic :.

1- Flavoring: Custard powder contains many different flavoring and vanilla extract
2- Coloring: Custard powder will enhance the color of the food
3- Thickening: Custard powder contains cornflour which can be use to thicken the sauce

.: Usage :.

Custard powder is broadly use in Western dessert.  For example, it can use to make the filling of the pastries, cakes, breads, pudding.
Hong Kong style Cuisine like to use custard powder too.  A lot of Dim Sum will use custard powder like egg tarts or some steam sponge cakes.  It is also used to make the lemon sauce or orange sauce
However, over use the custard powder will destroy the flavor of other ingredient, especially the seafood.