In a large saucepan, melt butter over medium heat.
Once the butter is melted, add the sweetened condensed milk, corn syrup, and brown sugar. Stir the ingredients using a whisk or rubber spatula.
Continue stirring the mixture over medium heat until it begins to simmer. Be sure to scrape the sides and bottom of the saucepan regularly to prevent sticking.
Allow the sauce to simmer until the temperature reaches between 200-205 degrees Fahrenheit. This process ensures that the sugar granulates properly but doesn't make it too thick like a soft ball stage candy.
Once the desired temperature is achieved, add the vanilla extract, cinnamon whiskey, and salt. Stir for an additional minute ensuring everything is well combined.
Remove the saucepan from the heat and allow the sauce to cool.
Once cooled, transfer the sauce into jars for storage. Optionally, add red hot candies for extra flavor.