Add milk, cocoa powder, nutella, and chocolate chips to a 4qt slow cooker. Whisk to combine, cover, and cook on LOW for 2 hours... stirring occasionally.
Serve topped with desired toppings! If adding liquor, add it to individual mugs, rather than the whole slow cooker, so the alcohol doesn't cook off.
Notes
STOVE TOP VERSION
Add milk, cocoa powder, nutella and chocolate chips to large saucepan or stockpot. Heat over MED heat and stir until everything is well combined and mixture is simmering. Simmer 5 minutes, then serve with desired toppings.