Combine chestnuts and pecans in a saucepan and toast over medium heat. Toast the nuts for 3-5 minutes until they start to smell toasty. You don't want to burn them, so keep an eye on them and stir often so all the nuts are evenly toasted.
Once the chestnuts and pecans smell toasty and fragrant, add in the water, sugars, and salt, and stir to combine.
Bring the mixture to a boil over high.
Once boiling, lower the heat to medium low and let simmer for 10 minutes.
Remove the mixture from the heat, add in the Pralines and Cream flavoring, and stir to combine.
Pour the syrup through a strainer into a mason jar or heat proof container, catching the pecans and chestnuts inside the strainer. Pro tip: You can add the nuts to a parchment lined baking sheet and bake at 375 degrees Fahrenheit for 10-15 minutes to make candied pecans/chestnuts.
Let the Starbucks Chestnut Praline Syrup cool completely before transferring to an airtight container, and store in the fridge for up to three weeks.