Pour the whole milk, heavy whipping cream, and sweetened condensed milk into a 4-quart or larger slow cooker.
Add the semi-sweet chocolate chips, cocoa powder, vanilla extract, and salt.
Whisk the mixture vigorously until the cocoa powder is mostly incorporated.
Cover and cook on LOW for 2 hours.
Whisk the mixture every 30 to 45 minutes to ensure the chocolate melts evenly and does not settle at the bottom.
Once hot and smooth, switch the slow cooker to the WARM setting.
Ladle into mugs and serve immediately with your choice of toppings.