Combine water and both sugars in a small saucepan over medium heat. Stir until the sugar dissolves.
1/4 cup brown sugar, 1/4 cup granulated sugar, 1/2 cup water
Reduce the heat to medium-low then add in the sweetened condensed milk, pumpkin puree, pumpkin pie spice, and salt. Stir to combine.
2/3 cup pumpkin puree, 14 oz sweetened condensed milk, 3/4 teaspoon pumpkin pie spice, 1/4 tsp salt
Bring the mixture just to a gentle simmer (do not let it boil hard) and simmer for 3 minutes, stirring often.
Remove from the heat, add in the vanilla extract, and mix again. Allow the sauce to cool down a little before adding it to a jar. Cool down fully before chilling in the fridge.
1 tsp vanilla extract
Notes
Do not let it boil hard. A gentle simmer is all you want. Aggressive heat splits the sweetened condensed milk and you get a grainy sauce instead of a smooth one. If it does split, pull it off the heat and whisk hard for about thirty seconds. It usually comes back.It will look too thin in the pan. It always does. This sauce thickens as it cools and it will be noticeably thicker after a few hours in the fridge. Do not simmer it longer to fix this or you will end up with pumpkin caramel.How much to use. Start with 2 tablespoons per drink and adjust up based on your taste preference or size of your drink. One batch makes about 20 drinks.Pumpkin puree, not pumpkin pie filling. Pie filling is already sweetened and spiced, and it throws the balance off completely.Storage. 2 weeks in an airtight container in the fridge, or 3 months in the freezer. It separates when it thaws, so shake or stir before using. A squeeze bottle makes weekday mornings much easier than a jar.Dairy free. Swap the sweetened condensed milk for sweetened condensed coconut milk. Slightly less creamy with a mild coconut note, but it thickens the same way.