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.
GIFT IDEA: You can put 1/4 cup of the hot chocolate mix into a 4-ounce jar and top it off with marshmallows to make small little gifts or party favors. (See video.)
TO RECONSTITUTE: To prepare your hot chocolate, add 1/4 cup of the mix to a mug and add approximately 8 ounces of boiling hot water or hot milk to the mug. Stir vigorously for about 60 seconds until the mixture is completely dissolved. Top with marshmallows, if you wish. Enjoy!