Place the cinnamon, cardamom, and vanilla bean, if using, on a square of 100-percent-cotton cheesecloth. Bring up corners of cheesecloth and tie with 100-percent-cotton string.
In a 3 1/2- to 4-quart slow cooker stir together the cream, milk, and baking pieces. Add spice bag. Cover and cook on low-heat setting for 4 to 5 hours or on high-heat setting for 2 to 2 1/2 hours, stirring halfway through cooking time. Remove spice bag and stir in vanilla, if using. If desired, top servings with grated chocolate.