To make the pumpkin spice syrup, add pumpkin puree, water, sugar, and spices to a saucepan. Bring it to a boil and stir frequently. Once the mixture is boiling and smooth, remove from the heat and let it sit for 15 minutes. Strain the syrup into a container and let it cool completely before making your drink.
For the cocktail, add the pumpkin spice syrup, lemon juice, and bourbon to a cocktail shaker and give it a good scare. Fill a glass with ice and strain the cocktail into the glass. Top with Potter's and you're ready to drink!