Sift the icing sugar, cocoa and cinnamon into a large bowl. Thoroughly whisk in the skim milk powder; stir in the marshmallows and chocolate chips. Store in an airtight container at cool room temperature; will keep for several weeks.
To make a serving of hot chocolate, spoon 1/3 cup of the mix into a mug. Pour in 1 cup of boiling hot water or scalded milk, stir to well combine and serve.