I think you will find your cocoa powder comes together better if you boil a bit of water on the stovetop. Put your cinnamon, cocoa, and sugar in dish and set aside. Boil your 4 or 5 tablespoons of water, then turn off the heat. Immediately add your cocoa, cinnamon, and sugar to the water, and whisk until smooth. Then, poor that into the milk you heated in the microwave. This eliminates cocoa lumps.