Add 1 1/2 cups white chocolate chipsor chunks to the bowl of a food processor. Pulse until the chocolate is finely chopped and resembles coarse sand. Scrape the edges of the food processor, especially the bottom to make sure the chocolate is not stuck there.
Add 3 cups skim milk powder, 1 1/2 cups Dutch-processed cocoa powder, 1 cup powdered sugar, and 1/4 teaspoon salt to the food processor. Process until the mixture is an evenly mixed, light brown powder.
Transfer the mixture to a large airtight jar or canister.
To make Hot Chocolate
Scoop 1/4 cup of the mix to a mug and add 1 cup milk. Warm in the microwave for 1 minute. Remove the mug and stir, heat for an additional 30-60 seconds as needed, and stir again. Top with marshmallows or whipped cream if desired.
Notes
Instead of heating your hot chocolate in the microwave, you could also heat the milk in a small saucepan over medium heat on the stovetop. Or you could use an espresso machine milk steamer. In both of the above cases, heat the milk on its own first before stirring in the hot chocolate mix.