Start by heating a small saucepan over medium heat, then add the sugar and salt, followed by enough water to cover them.
Bring the mixture to a gentle simmer for about 5 minutes, stirring occasionally with a silicone spatula until the sugar has completely dissolved.
Turn the heat up to medium-high and cook the caramel without stirring until it turns a rich golden color.Use a silicone brush dipped in water to wipe down the sides of the pan, preventing any sugar crystals from forming.If you’re using a candy thermometer, aim for a temperature of 350°F.
Once the caramel reaches a deep copper shade, remove it from the heat and carefully stir in the heavy cream and butter.
Let the caramel cool slightly in the pan, then quickly pour it over the cooled pastry before it begins to set.Allow it to cool just until it’s safe to touch — it will start firming up fast.