Place sugar in medium saucepan and heat to medium. Stir occasionally with a silicone spatula (make sure it can handle high heat temps!). As the caramel begins to liquefy, scrape from the bottom and mix back into the granulated pieces. The sugar will clump but eventually it will melt again and turn an amber brown. Make sure you are still stirring so it does not burn.
Once the sugar is melted, add butter and stir until butter is melted, about 2 minutes. Use a whisk to incorporate the butter with the sugar. Keep an eye on the color of the caramel. If it's getting too dark, remove from heat to cool down, and then add back to heat.
Very, very slowly, add the cream to the sugar mixture while whisking. The mixture will start to bubble violently, but keep stirring and it will calm down. Allow it to boil for 1 minute.
Remove from heat and stir in the salt. Transfer to storage container (such as a mason jar) and allow it too cool for at least 1 hour before using.