Add sugar and water in a heavy bottom pan. Make sure the pan is tall, this is because when you add cream/butter, the mixture bubbles up like crazy. So you need a pan with tall sides. Set the heat to medium-high.
The sugar will dissolve and the mixture will start boiling.
Dip your pastry brush in some water and keep brushing down the sides of the pan. This will prevent sugar crystallization. Keep brushing the sides from time to time.
The mixture will keep boiling, do not whisk it in between or stir it. Just let it boil and keep brushing the sides to prevent crystallization.
After around 13-15 minutes, the sugar will caramelize to a deep brown color at 350 F degrees. [See picture above]. If you don't have a kitchen thermometer, that is okay, just keep a close eye on the color. Also, the smell gives it away, however be careful and distinguish between the smell of caramel and burnt sugar. This happens really quickly, once the mixture reaches 320 F degrees it kind of goes very quickly from there.
As soon as the sugar reaches 350 F degrees, remove the pan from heat and after 15 seconds add butter to it. The mixture is going to bubble up so wear oven mittens and whisk like crazy. Then add the cream and whisk like crazy again till it's all smooth.
Now this is optional but I like to put the pan back on heat for a 30-45 seconds, just to kind of thicken it up.
Add vanilla extract and whisk. Remove from heat, stir in the sea salt and whisk.
Let the sauce cool completely and then store in the refrigerator. The sauce will solidify in the fridge, so whenever you want to use it just take it out of the refrigerator, microwave for few seconds and it's ready to be used in anything you like.