Combine all the dry ingredients in a large bowl and mix well. Take the bittersweet chocolate and roughly chop it up to give a chunky but consistent size. Mix well with a wooden spoon and then store in a dry airtight container. To make the hot chocolate, simply add 1 cup of water per 1/2 cup of hot cocoa mix. Heat in a small saucepan over low heat, stirring with a whisk. Heat until hot, but do not boil, about 6 to 7 minutes. Serve with marshmallows.