Pour the sugar into your sauce pan and put it over medium heat. Whisk gently for a few seconds every 30 seconds or so, to avoid letting it burn. The sugar will slowly melt and clump up; keep going until it's all melted. This will probably take around five minutes, but don't go by time, go by the look of the sugar.
Immediately upon the sugar fully melting, remove it from the heat. It can go from melted to burned very quickly if you aren't careful.
Add the room temp butter. The hot sugar and comparatively cool butter will bubble and sizzle, but don't freak out! Stir it in and it will settle down.
Once the butter is fully mixed in, add the heavy cream and salt, adjusting the salt for how salty you want the caramel to be. 1 tsp is enough for a light saltiness, but not an overriding flavor. You can also use kosher salt for the occasional bit of salt crystal, if you prefer.
Stir the mixture until it's well combined, then pour it into a jar for storage. Let it cool on the counter for a bit, and store in the fridge. To use, just take a spoonful as it is, or heat it in the microwave for about 7 seconds to thin it back out.