Prep the crockpot: Lightly spray or butter the inside to prevent a chocolate ring. This makes cleanup easier.
Add the base: Pour in the milk and cream.
Whisk in the cocoa powder, sugar, cinnamon, chili powder, cayenne, and salt until no dry pockets remain.
Stir in chocolate: Add the chopped chocolate and vanilla. Give it a quick stir to distribute.
Cook low and slow: Cover and cook on Low for 2 to 2 1/2 hours, or on High for 1 to 1 1/2 hours. Stir every 30 minutes to help the chocolate melt evenly and prevent a film on top.
Adjust to taste: When the chocolate is fully melted and the mixture is smooth, taste.
Add more sugar for sweetness or a pinch more cayenne for heat. If it’s too thick, whisk in a splash of milk.
Keep warm: Switch to Warm. Stir before serving and ladle into mugs.
Add toppings as you like.