In a 3-quart (or larger) slow cooker, combine the sugar, cocoa powder, milk, half-and-half, and vanilla. Whisk until all of the cocoa powder is dissolved and fully incorporated. Stir in the chopped chocolate.
Cover and cook on LOW, whisking every 45 minutes, for a total time of 2 to 3 hours (cook until all of the ingredients are melted and well-combined and the drink is hot). Reduce the heat to WARM while serving. Garnish with marshmallows or whipped cream.
ADDITIONAL FLAVOR VARIATIONS Salted Caramel Hot Chocolate:Make a batch of Crockpot Hot Chocolate as directed. Stir in 1/2 cup caramel sauce + 1/4 teaspoon fine sea salt until dissolved and fully incorporated (feel free to add more caramel and/or salt, to taste). Garnish with fresh whipped cream, a drizzle of caramel sauce, and flakes of sea salt. (Or if you prefer plain ol' caramel hot chocolate, just leave out the salt.Peppermint Hot Chocolate:Add ingredients to the crockpot along with 1/2 teaspoon pure peppermint extract. After the Peppermint Hot Chocolate is finished cooking, taste and add extra peppermint extract if desired, 1/4 teaspoon at a time. Be careful and add gradually, tasting in between, as peppermint extract is very strong and can go from the perfect amount to way too much very quickly! Garnish with peppermint marshmallows or fresh whipped cream and crushed candy canes. (For a boozy version, swap out the peppermint extract for peppermint schnapps…add 1/2 cup schnapps at a time until it tastes just right! You’ll probably end up needing around 1 cup total.)Mexican Hot Chocolate:Add ingredients to the crock pot along with 1 tablespoon instant espresso powder, 1 teaspoon ground cinnamon, + 1/4 teaspoon ground nutmeg. If you have Mexican vanilla, use it in place of regular vanilla extract. Garnish with cinnamon sticks. (For optional heat, add 1/8 teaspoon chipotle chile pepper powder at a time until you’re happy with the level of smoky spice. And for a boozy version, you can even stir in a little Kahlua.)