Cut chocolate into small pieces ~ 1 cm in thickness. Place the chocolate into a large mug.
Add cocoa powder to the mug.
Heat the milk. You can heat it in the microwave. Every microwave is different at heating. Try heating it for 45 seconds - check the heat, and put it in for another 30 seconds if necessary. You could also heat the milk in a small saucepan over low to medium heat. Once the milk comes to a simmer, remove it from the heat source and pour the milk into your mug only 1/3 full.
Let the milk melt the chocolate. Leave it for a minute or two, then start stirring. Stir until the cocoa powder is completely dissolved. By only adding 1/3 of the milk, you can mix vigorously without spilling.
Add the rest of the milk and mix well until all of the chocolate has melted.
Top your hot chocolate with a few marshmallows, whipped cream, and few sprinkles. Serve immediately.