Add the brown sugar, water, pumpkin puree and pumpkin spice mix to a small saucepan.
Put the saucepan over medium high heat. Stir until it comes to a boil.
Lower the heat until the mixture is just simmering. Let it simmer for about 3 minutes.
Take the saucepan off the heat and let it cool to room temperature. (You can speed up the cooling by placing the bottom of the saucepan in an ice bath and stirring.)
Once it's cool, pour the syrup into your storage bottle using a funnel.
Seal up the glass bottle. You can use it right away, or keep it in the fridge for up to three weeks.