In a saucepan, heat the milk over medium heat, stirring occasionally.
Add unsweetened cocoa powder and sugar to the milk.
Whisk until the cocoa and sugar are fully dissolved.
Once the milk is steaming, add the chopped semi-sweet chocolate.
Stir continuously until the chocolate melts completely.
Remove from heat and stir in vanilla extract.
Pour the hot chocolate into mugs.
Top with whipped cream or marshmallows.
Drizzle with additional chocolate syrup if desired.
Serve immediately and enjoy!