Add the milk powder, corn flour, food colouring and vanilla to a food processor or blender, then blend for about 10 seconds.
Transfer to an airtight container and store in the pantry.
TO TURN IT INTO CUSTARD
Add 1/2 cup of custard powder to a small saucepan with the sugar and milk. Whisk to combine, then heat over low heat stirring constantly until thick and beginning to bubble. Add more milk if you want a thinner consistency.
I use a standard 20ml Australian tablespoon (= 4 teaspoons worldwide)
Milk powder: You can use skim or full cream versions
Vanilla: I like to use vanilla sugar as it blends in easier. You can use vanilla extract but you will get a few lumps. This isn't a problem - just try not to blend it too long as the warmer the mixture gets, the bigger the lumps get and just make sure any lumps are well dispersed in the mix. This way, when you scoop out the powder to use it, each batch will still have vanilla in it. You can also use vanilla beans straight from the pod.
Milk in the custard: Whole / full cream milk will give a creamier sauce, however you can use light or skim milk or even water if you'd like a lighter version
Storage: Store in an airtight container in the pantry. Dry mix will last as long as the expiration on your milk powder. If you use vanilla extract or beans, you should try to use the custard powder within a month.