Spray a medium pot with non-stick cooking spray, which prevents the sugar crystals from sticking to the sides of the pot.
Add the sugar and water over low heat. Whisk until the sugar is dissolved and then stop whisking completely and turn the heat up to medium.
Don't touch the sugar at all and watch as it turns from white to pale yellow and then dark amber and golden brown. Keep a close eye on it because the sugar can go from perfectly caramelized to burnt in a matter of seconds. If the sugar does burn you have to start over. It usually takes about 10 minutes for it to get to the perfect amber color you are looking for in caramelization. Gently swirl the pot if the sugar is browning unevenly.
Remove from heat and immediately whisk in the heavy cream. Do this carefully and in a slow and steady stream, the caramel will bubble up violently at first and then settle once the temperature evens out.
Add in the butter and salt and cook over low heat until the butter is melted. Remove from heat and allow the caramel sauce to cool and thicken before pouring it over the chocolate ganache.
Pour the cooled caramel sauce over the cooled chocolate ganache. Allow the chocolate caramel tart to cool and harden completely before slicing and serving. At least 2 hours or overnight.
Cover leftovers tightly and keep refrigerated for up to 1 week.