I came from England, and missed custard powder.  When I wanted to make custard for Trifle at Christmas time I came up with this substitute.  I hope it helps.

Custard

Cornstarch  6 TBSP

Sugar scant half cup

Salt 1/8 tsp

Milk 2 cans of evaporated milk + enough water to make 4 and 1/8 cups total

Butter 3 TBSP

Vanilla 1 1/2 tsp

Mix together the sugar salt and cornstarch in a saucepan, gradually stir in the milk till there are no lumps in the cornstarch.  Then cook over moderate heat stirring (milk scorches easily) until the mixture starts to come to a boil.  Boil and stir about 2 mins. The mixture should have thickened some.  Then take the pan off the heat and stir in the butter and vanilla.  You can make it with plain milk but it will not have the nice creamy taste.  Hope this helps.  Joyce

Just a note: The custard powder used in the recipe is dry…no need for the evaporated milk or water!