We make up batches of our own mix:
2 cups icing sugar (sieved)
2 cups whole milk powder
1 cup raw cacao powder (sieved) - can use unsweetened cocoa instead
A pinch of salt
You can add a couple of tbsp of cornflour to help thicken it but we don't find you need it, and it makes the texture a bit artificial.
Mix together really well. It keeps in an airtight jar for months and is delicious, we use 4 heaped tsp mix per mug of milk