Fill a large bowl with powdered milk.
Using a fine-mesh strainer, add cocoa powder to dry milk powder. Using the strainer will help to make sure that there are no lumps in the cocoa powder.
Add salt to the bowl.
Add arrowroot powder to the bowl, if using.
Using a small grinder or blender, grind the sugar into a powdered form.

Add the "powdered" sugar to the bowl.
Mix all ingredients together very well using a whisk until fully incorporated.
Transfer the hot chocolate mix to a quart-sized jar to store. The shelf life of the mix will be approximately two years if you used non-fat powdered milk. The shelf life will be decreased to six months to one year if you used full-fat whole milk powder.

#Note:

You can use instant non-fat dry milk powder or whole milk powder can be used.

Whole Sweetener such as Sucanat, ground into a powdered form You can also use other whole sweeteners, including maple sugar, coconut sugar, or date sugar. White sugar can also be substituted.

If you don't have cornstarch use arrowroot powder or white rice flour can be substituted.