Add the sugar in an even layer over the bottom of a medium to large 4-quart heavy-bottomed saucepan. Heat the sugar over medium heat, whisking it carefully as it begins to melt. The sugar will begin to form clumps as it melts. Keep whisking as it continues to cook, until it is completely melted (no clumps) and has turned an amber color, and developed a toasted aroma. This is where caramel can go from good to burnt in a matter of seconds, so watch it carefully and use your instinct.
As soon as the caramel reaches the right color and consistency, carefully add the butter all at once. The caramel will bubble up and foam when the butter is added, but will calm down once you start whisking. Whisk the butter into the caramel until it is melted. Remove the pan completely from the heat.
Slowly pour the cream into the caramel, while whisking. The mixture will once again bubble and foam. Whisk until all of the cream has been incorporated and you have a smooth runny sauce. Add the Maldon Salt and whisk to incorporate. You can always add more salt later if desired, at the end.
Set the sauce aside to cool in the pot for a few minutes and then transfer into a measuring cup so that you can transfer to your preferred glass jar(s) for storage. Let cool to room temperature. It will thicken as it cools to room temperature, and thicken even more once it's stored in the fridge. The sauce can be refrigerated for up to 3 weeks.