First, preheat a small saucepan over medium heat, and add in the sugar and salt, and cover with water.
Next, bring the mixture to a simmer for about 5 minutes or until the sugar is completely dissolved, stirring only occasionally with a silicone spatula.
Then, increase the heat to medium-high, and continue to cook the caramel mixture, without stirring, until it reaches a deep golden color.Brush the inside edges of the saucepan with a silicone brush and some water to avoid the caramel hardening on the sides while it simmers.If you have a candy thermometer, the caramel should reach 350°F.
Once the caramel is a deep copper color, turn off the heat and immediately stir in the heavy cream and butter.
Finally, let the caramel mixture cool just slightly in the pan, then immediately pour it on top of the cooled pastry before it starts to harden. Remember to let the caramel cool just to the touch, since it will begin to harden pretty quickly.