Add parchment paper to a 9x9 pan and set it aside. 

In a high-sided pot over medium-high heat, add granulated sugar, light corn syrup, and kosher salt.

Stir the mixture to help dissolve the sugar into the corn syrup. Make sure to keep slowly stirring as you do not want any hot spots in the pan to burn the sugars. 

Once the mixture starts to gently bubble, and the granulated sugar has dissolved, VERY SLOWLY drizzle in the heavy creamy while continually stirring. You do not want the mixture to stop bubbling, so you must drizzle this cold heavy creamy in very slowly.

When all the heavy cream has been added, add in the butter while continually stirring. 

Continue gently stirring, until the mixture has reached 245°F, on a candy thermometer or instant-read thermometer, about 15-20 minutes. 

Once the mixture has reached 245°F, remove from the heat and quickly stir in the rum extract. Immediately pour the hot caramel into the prepared pan. 

Let it sit on the counter for a few hours to set up and slightly harden. 

Once cooled and caramel is set up, remove the parchment from the pan, cut the caramel into squares, wrap it in wax paper, and enjoy!