Cover and cook on HIGH for 2.5 hours. Stirring occasionally.
Serve topped with whipped cream or marshmallows.
Video
Notes
If you can't find pumpkin pie spice I would personally start with 2 teaspoons of cinnamon, 1/2 tsp of nutmeg and a dash of cloves. After the cocoa has heated, taste and add more of what you think it needs.
This is a full pot of hot chocolate, it would be hard to travel with all made up. I suggest making this at your party location. You can heat up the milk in the microwave a bit in a glass container to speed up the process.
Want to make pumpkin white hot chocolate? Use 3 bags of white chocolate chips instead of the milk chocolate chips, and skip the cocoa powder.