3cups(405g)dry nonfat or whole milk powder (see note)
1 ½cups(128g)cocoa powder, dutch-process or natural unsweetened (see note)
1 ½cups(255g)finely chopped white chocolate or white chocolate chips (see note)
Whisk together all ingredients in a large bowl. Working in two (or more) batches, depending on the size of your food processor, pulse the ingredients in a food processor until the chocolate is finely ground. Store the dry mix in an airtight container for up to 3 months. (If your food processor is large enough, ditch using an extra bowl and throw all the ingredients in the food processor at once.)
To make hot cocoa, put 1/3 cup of the cocoa mix in a mug and stir in 1 cup of hot milk or almond milk (water works ok but the hot chocolate isn't as creamy or flavorful). Top with whipped cream or miniature marshmallows, if desired.
Milk Powder: my favorite dry milk powder is a whole milk kind I get on Amazon here, but I also use nonfat dry milk powder when I'm out of whole milk powder (favorite brands are Bob's Red Mill or Carnation). After years of making this stuff, it's important to note that quality/taste matters with all of the ingredients. Brands: if you are using a milk powder with a fake/strange after taste, that's going to come through in the hot chocolate. Same with cocoa powder and white chocolate. Use a brand that you love - my favorite batches of this mix are made with part natural, unsweetened cocoa powder (Hershey's brand, usually) and part Dutch-process cocoa (Ghirardelli dark cocoa powder, Droste brand, Trader Joe's brand, etc). White Chocolate: I do often use white chocolate chips when I don't have white chocolate bars on hand, but they never mix quite as well in the food processor, so I recommend using the white chocolate bars, if possible (NOT almond bark or candy coating). The grocery stores in my area all carry the Ghirardelli white chocolate (here's a visual for reference), and it blends up nicely in this mix.