Pour the sugar into a large pot and heat it over medium heat, stirring with a kitchen whisk as it melts. If you use very coarse cane sugar, you can add 2 tablespoons of water to the sugar, but only if it is hard to melt.
At first, small lumps will form, but it does not matter; keep stirring, and slowly all the sugar will melt.
Try to keep the edges of the pot clean, possibly using a silicone spatula to move the sugar that settles along the sides of the pot.
When the sugar has melted, stop mixing it with the whisks and let it cook, lowering the flame slightly, until it turns a nice amber color. If you have a candy thermometer, use it to keep an eye on the temperature, which should not exceed 175°C.
When the caramel reaches the indicated temperature, add the butter, all at once. The caramel will bubble furiously for a few seconds (this is the reason why you have to use a tall, roomy pot), then the butter will melt and everything will go back to normal. Mix the caramel butter well with a whisk, then when all the butter is incorporated add the cream and stir again.
When all the cream has been incorporated you should have obtained a velvety sauce, to which you can add, if you want, the Fleur de Sel.
Allow the sauce to cool for about ten minutes, then pour it into a clean glass jar.
If by chance there are lumps of sugar left, which is not desirable, but still possible, you can put the sauce through a strainer, before transferring it into the container.
You can keep the caramel sauce in the fridge for up to two weeks, remembering that you should always heat it a little before serving it for any other recipes.