Heat cream in a saucepan, but do not bring to a boil.
Spread sugar evenly over the bottom of a large coated pan and heat over medium heat until it begins to melt and brown caramel flecks form on the surface without stirring it.
Once the sugar begins to melt, gently stir it with a wooden spoon over and over until it has completely dissolved into a light caramel-colored liquid, and remove the pan from the heat immediately after. (Be sure to follow the detailed tips and tricks for making your own caramel sauce here at the top of this post).
Immediately add butter and whisk until melted and a homogeneous mixture has formed.
Add salt and stir it briefly.
While stirring constantly, gradually add hot cream (it must be about 90 °C) in small portions and stir it until a homogeneous caramel mass is formed. (If a few lumps have formed in the process, it's not a problem. They will dissolve during cooking).
Place the caramel mixture back on the stove, bring to a boil, stirring constantly, and then simmer, stirring, for about 4 minutes, until it reaches 108°C.
Remove the pan from the heat, immediately pour the caramel sauce into a clean mixing bowl or screw-top jar, and let it cool. Once cooled, the liquid caramel sauce will become more solid.