In a medium saucepan, combine the brown sugar, syrup, butter, and ¼ cup (60 ml) water. Clip a candy thermometer to the saucepan and set the pan over medium-high heat. Cook until the mixture reaches 245°F (118°C).
Add the cream and stir to combine, being careful not to let the hot syrup spatter on you. Halve the vanilla bean and scrape out the seeds. Add the seeds and pod, the whisky, and the salt to the pan and cook for 5 minutes more.
Remove the pan from the heat and let cool for 10 minutes. Use a fork to remove and discard the vanilla bean pod. Add the vanilla extract and stir to combine.
Serve immediately or store in an airtight container in the refrigerator for up to 2 weeks. (The sugar may start to crystallize, but if you reheat it, you can stir until the sugar crystals dissolve.)