Heat the pumpkin, pumpkin spice, and brown sugar in a sauce pan over medium heat for a few minutes. Continue heating until the pumpkin is hot and fragrant.
Slowly whisk in the vanilla and continue whisking until the ingredients are well combined.
Next, slowly whisk in the milk.
Continue to simmer the milk until it's hot and steamy, but be careful, if the heats to high it will foam up and make a mess. Once the milk is hot and steamy vigorously whisk it until it's frothy. You can use a blender if you can't get the mixture frothy enough for your liking.
Pour a shot of coffee and Grand Marnier into a coffee cup. Top with the milk mixture until the glass is full. Stir.
Finally garnish (optional) with whipped cream, a cinnamon stick, and a sprinkle of pumpkin pie spice.