Prep the crockpot: Set your slow cooker to Low. Lightly spray the insert or rub with a bit of butter to prevent sticking.
Mix dry ingredients: In a small bowl, whisk cocoa powder, sugar, cornstarch, cinnamon, chili, and salt.
This helps prevent lumps.
Add liquids: Pour milk and cream into the crockpot. Whisk in the dry mixture until well combined.
Add chocolate: Stir in the chopped bittersweet chocolate.
Cook low and slow: Cover and cook on Low for 2 to 2.5 hours, stirring every 30 minutes to melt the chocolate evenly and prevent a skin from forming.
Finish with vanilla: When the mixture is smooth and steamy, stir in the vanilla. Taste and adjust sweetness or spice.
Serve: Ladle into mugs and garnish with whipped cream, a cinnamon stick, or chocolate shavings.
Keep on Warm for serving.