In a small saucepan, combine the cream and salt. Bring to a boil, then remove from heat and set aside until ready to use.
In a medium-sized saucepan, add the sugar in an even layer and gently pour the water on top. Without stirring, cook the sugar over medium heat until the sugar mixture comes to a boil and begins to caramelize. To prevent crystallization, use a pastry brush dipped in cold water to remove any sugar crystals that may form around the side of the pan while it cooks if needed.
Once the sugar begins to turn golden amber in color, gently swirl the pan to evenly distribute the caramelization.
Allow the caramel to become a deep amber color, then remove the pan from the heat and immediately whisk in the warm cream mixture (it will bubble up at first, but will calm down as it’s whisked).
Whisk in the butter pieces until fully incorporated. Place back over medium heat, whisking constantly, and cook the salted caramel until it reaches 118C on a candy thermometer.
Remove from the heat and whisk in the vanilla. Pour the salted caramel over the top of the shortbread and let cool completely to fully set.