In a medium saucepan, whisk together the milk, heavy cream, and pumpkin puree over medium heat.
Add the cocoa powder and whisk until smooth and no lumps remain.
Add the chopped dark chocolate, brown sugar, vanilla extract, pumpkin pie spice, and salt.
Cook for 5–7 minutes, whisking frequently, until the chocolate is melted and the mixture is smooth and hot. Do not let it boil.
Remove from the heat and pour into mugs.
Top with whipped cream and an additional sprinkle of pumpkin pie spice, if desired.
Notes
For an extra-smooth texture, blend the hot chocolate with an immersion blender before serving. For the best balance of sweetness and chocolate flavor, use dark chocolate with no more than 70% cocoa. If using chocolate with a higher cocoa percentage, adjust the brown sugar to taste.