Preheat the oven to 350 degrees F. Place the chocolate graham crackers in the food processor. Pulse the crackers into fine crumbs. Pour the melted butter into the food processor and pulse again until well combined.
Press the chocolate crumbs, firmly and evenly, into a 9- or 10-inch tart pan. Make sure to press the crumbs up the sides. Bake for 10 minutes.
Meanwhile, place sugar and water into a medium sauce pot for the caramel filling. Fit a candy thermometer on the edge of the pot, and turn the burner on medium-high. Bring the sugar to a boil and let it simmer until it turns dark and reaches between 330-350 degrees F. The color should be somewhere between a medium honey brown and a dark sweet tea brown. Do not stir the sugar syrup, but swirl the pot on occasion. Once your desired color is reached, immediately take it off the heat. (Lighter colored sugar offers a lighter caramel taste, whereas dark syrup creates a bold deep caramel flavor.)
While the sugar is boiling place the heavy cream, butter, vanilla, and salt in a microwave-safe bowl. Warm the ingredients for 1-2 minutes in the microwave, until the butter melts. Then stir well. (This can also be done on the stovetop.) Once the syrup is taken off the heat, quickly whisk the cream mixture into the caramelized syrup, until very smooth.
Pour the caramel filling into the baked tart crust. Tap it on the counter gently to remove bubbles. Then place the tart in the refrigerator to set. Allow at least 30 minutes of chill time, before adding the ganache layer.
For the chocolate ganache, break the chocolate into pieces and place in a microwave-safe bowl. Pour the cream and corn syrup over the chocolate. Heat in the microwave, in one-minute increments, until the chocolate is melted and stirs smoothly into the cream. (This can also be done on the stovetop.) **The corn syrup is optional. It makes the ganache shiny, but is not necessary for flavor or texture.
Pour the chocolate ganache evenly over the top of the caramel filling. Use a spatula to spread the chocolate to the edges. Place in the refrigerator and chill for at least one hour. Then sprinkle the top with salt flakes.
You can keep the tart in the fridge. However for best cutting results, I like to keep the tart in the freezer. For neat edges, lift the tart out of the ring and cut the pieces while frozen. Thaw the pieces on dessert plates for 15 minutes before serving. This Salted Caramel Chocolate Tart can be made up to a week in advance!
NOTES: You can make this recipe using standard wheat-based chocolate graham crackers or wafer cookies. Yet to make it gluten-free, find gluten-free cookies or grahams. Pamela's makes a nice GF graham cracker. If you can't find them, you can also use Annie's GF chocolate bunny grahams, or GF Oreo-style cookies.