In a saucepan, combine cocoa powder, sugar, and cornstarch. Pour in milk and whisk until cornstarch is fully dissolved.
Bring to boil over medium heat, whisking constantly until the mixture just begins to bubble but does not boil.
Reduce heat to low then add chopped dark chocolate and keep stirring until melted. Pour in the hot chocolate into a mug and garnish it with marshmallow. Enjoy!