Warm the base - Add the whole milk, heavy cream, and sweetened condensed milk to a large pot over low medium heat. Stir to combine.
Add the chocolate - Mix in the chopped dark chocolate (or chocolate chips) along with the cocoa powder. Whisk until everything melts and the mixture becomes smooth and rich.
Flavor it - Add the vanilla extract and continue cooking on low heat for 5–10 minutes, stirring frequently. Don’t let it boil—just keep it warm and creamy.
Serve - Pour into mugs and top with whipped cream, marshmallows, chocolate syrup, or your favorite toppings.