Combine water & sugar: Stir the sugar and water in a sauce pan until it's combined and there are no sugar lumps.
Place on heat: Place the pan on medium heat, and let it dissolve. Leave the mixture and DO NOT stir, otherwise it will crystalize.
To avoid crystallization: Use a pastry brush dipped in water to brush away the sides of the pan to melt sugar crystals above the water line.
Let sugar cook: it will start to bubble up and slowly change colors. This could take some time, but keep a watch on it because it could burn very fast once it starts to change
Swirl the pan around: When you start seeing color around the edges, at this point I like to take the handle of the pan and give it a quick swirl so that the caramel color starts becoming even all around. At this point, it will start to turn pretty quick so keep a close eye on it.
Add heavy cream: Once it reaches desired color (an amber color is good. Any darker and it could become bitter), remove the pan from the heat and slowly add the room temperature heavy cream, and whisk as you pour the cream (don't add too quickly, otherwise it could seize). Whisk until it all comes together and smooth. Place it back on the heat and let it cook, while whisking, for another minute.
Remove from heat & add butter: Once heavy cream is mixed in, add room temperature butter and the salt, and continue whisking until incorporated and melted into the caramel
Pour into jar or container and let cool.
Store: Cover and store in the refrigerator. To use, reheat small amounts, or all if using it all at once. You can reheat in the microwave in 30 second increments.