Step 1: Lightly grease the inside of your Crockpot with a bit of butter or non-stick spray, if desired, to prevent sticking.
Step 2: Pour the whole milk and sweetened condensed milk into the Crockpot. Whisk them together until well combined.
Step 3: In a separate medium bowl, whisk together the unsweetened cocoa powder and granulated sugar. This helps prevent lumps.
Step 4: Gradually add the dry cocoa mixture to the milk in the Crockpot, whisking continuously until the mixture is smooth and no lumps of cocoa powder remain.
Step 5: Stir in the semi-sweet chocolate chips (or chopped chocolate) into the liquid mixture.
Step 6: Cover the Crockpot and cook on LOW for 2-3 hours, or on HIGH for 1-1.5 hours. Ensure the hot chocolate is steaming hot and the chocolate chips are fully melted, but do not allow it to boil.
Step 7: Once heated through, give the hot chocolate a final, thorough whisk to ensure everything is perfectly combined and smooth. Stir in the pure vanilla extract.
Step 8: Serve immediately into mugs. Keep the Crockpot on the "warm" setting for continuous serving. Garnish with your favorite toppings like whipped cream, marshmallows, or chocolate shavings.