Pour the 8 cups of whole milk and the 2 cups of heavy cream directly into the basin of a large, 6-quart or 7-quart slow cooker (Crockpot).

Carefully open and scrape the entire contents of the 14-ounce can of sweetened condensed milk into the milk and cream mixture. Use a rubber spatula to ensure all the thick milk is removed from the can.

Add the 1/2 cup of unsweetened cocoa powder and the 1/4 teaspoon of salt to the liquid mixture. Using a large whisk, thoroughly mix the ingredients until the cocoa powder is completely incorporated and no dry clumps remain floating on the surface of the milk. This initial whisking helps ensure a smooth final product.

Pour the 1 cup of semisweet chocolate chips into the slow cooker. Gently stir the mixture one final time to distribute the chips evenly throughout the liquid.

Place the lid securely onto the slow cooker. Set the temperature control to the LOW setting. Allow the mixture to cook for a minimum of 2 hours and a maximum of 3 hours.

After the first 90 minutes of cooking, remove the lid and use a sturdy spoon or ladle to stir the hot chocolate mixture thoroughly. Ensure that all the chocolate chips are fully melted and that the mixture is smooth and hot throughout. If any chips remain, replace the lid and continue cooking for the remaining time.

Once the hot chocolate is heated through and all ingredients are fully combined (around the 2 to 3-hour mark), turn the heat setting off or switch it to the KEEP WARM setting. Lift the lid and stir in the 1 tablespoon of vanilla extract. The vanilla should be added last for the best flavor infusion.

The Crockpot Hot Chocolate is now ready to serve. Use a ladle to transfer the hot chocolate into mugs. Garnish generously with marshmallows, a dollop of whipped cream, or a dusting of extra cocoa powder as desired. The hot chocolate can be kept warm for up to 4 hours.