Pour the chocolate milk into the bowl of the slow cooker.
Cook:
Cook on low for for 2 to 3 hours or on high for 45 minutes to 1 hour. Add the vanilla at the very end and stir.
When it's time to serve it, reduce the heat to the warm setting. If your slow cooker doesn't have a warm setting, reduce it to low and stir occasionally to prevent burning at the bottom of the pot.
Optional: Top the slow cooker with marshmallows and a dusting of cocoa powder.
Serve:
Laddle out the hot chocolate into mugs and top with marshmallows and chocolate chips or cocoa powder.
Notes
Notes: you can also use ½ gallon of chocolate milk for this recipe for 8 servings.