Add the half and half, condensed milk, and pumpkin puree to the bowl of the slow cooker. Whisk to combine.
Whisk in the pumpkin pie spice, cinnamon, and cocoa powder until well combined.
Place the lid on the slow cooker and cook on low for at least one hour.
When ready to serve, pour in the milk and whisk to combine. Place the lid back on the slow cooker and heat for 10 minutes, until the milk has warmed. Stir again before serving.
Top with whipped cream, marshmallows, and/or cinnamon before serving, if desired.