½cupunsalted butter (1 stick), cold, cut into cubes
1large egg yolk
For the Chocolate Ganache Filling:
1cupsemi-sweet chocolate chips
1cupmilk chocolate chips
For the Caramel Sauce:
Make the Tart Crust:
In the bowl of a food processor, pulse to combine the flour, salt, powdered sugar, and cocoa powder. Add in the butter and pulse until evenly coated. Then add in the egg yolk, vanilla extract, ice water, and heavy cream and pulse until a dough forms. Wrap the dough in plastic wrap and refrigerate for 30 minutes or overnight.
On a lightly floured surface, roll the dough out into a 12" circle. Press it into a 9" tart pan with a removable bottom and trim the edges. Prick the bottom several times with a fork and refrigerate for about 30 minutes before blind baking.
Blind bake the crust: Cover the crust with parchment paper and fill with pie weights or dried beans. Bake in a 350°F oven for 15 minutes. Remove the weights/beans and parchment paper and continue baking for another 10 minutes until the bottom of the crust is fully cooked.
Make the Chocolate Ganache:
Add both chocolate chips to a large bowl and set aside.
In a medium pot, heat the butter and heavy cream until the butter is melted and the cream is just starting to boil. Pour over the chocolate chips and whisk until the chocolate is melted and smooth.
Pour the ganache into the baked tart crust. Allow to cool and harden for at least 30 minutes or overnight before adding the caramel sauce.
Make the Caramel Sauce:
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.