Make the caramel: In a small pot, bring the heavy cream to a simmer. Remove from the heat and whisk in the corn syrup. Set aside.
In a second small saucepan, add the sugar and 1 tablespoon of water, swirling with your fingers to completely moisten the sugar, then wiping the edges of the pot with clean wet fingers to remove any sugar from above the surface of the syrup. Bring to a boil over medium-high heat, and cook, rotating the pan once or twice to ensure even cooking, until a dark caramel forms, about 3½ minutes or 370°F on a candy thermometer. Immediately remove from the heat and use a wooden spoon or heat-resistant spatula to slowly stir in the warm cream. Return the pan to the heat and cook, stirring constantly until the caramel is completely dissolved and the sauce is very smooth.
Remove the pan from the heat and immediately add the chocolate, butter, salt, and whiskey (if using). Stir until homogenous. Set aside at room temperature to cool completely.
Make the dough: In a medium bowl, sift together the flour and cocoa. Add the salt and set aside.
In a stand mixer fitted with the paddle attachment, beat the sugar and butter on medium speed until light and fluffy, about 2 minutes. Whisk together the egg and 2 teaspoons cold water and slowly add the egg mixture to the butter mixture, stopping to scrape down the sides of the bowl a few times and continuing to mix until completely homogenous, about 3 minutes more. Add the dry ingredients and mix on low speed, stopping to scrape down the sides as needed, until just combined. Remove the dough and press into a compact disk, ½ inch thick. Wrap tightly plastic wrap and refrigerate until firm, about 2 hours.
Retrieve the dough, unwrap, and place on a lightly floured piece of parchment paper. Working quickly, roll out the dough to an even 12½-inch disk, ⅛ inch thick. Slide the parchment paper and dough onto a baking sheet and refrigerate again until slightly firmed, 5–10 minutes.
Set a shallow 9-inch tart ring or tart pan with a removable bottom on a parchment-lined baking sheet. Carefully transfer the dough round to the ring or pan, gently pressing into the bottom and sides. Trim the edges to be flush with the edge of the tart ring. Poke a few holes across the bottom of the tart dough with a fork, then freeze for 30 minutes.
Meanwhile, preheat the oven to 325°F. Bake the tart shell straight from the freezer until it is just dry and fragrant, 12–14 minutes. Let cool completely in its ring or pan before filling.
When the tart shell is cool, pour ½ cup of the caramel sauce into the bottom; use an offset spatula or the back of a spoon to smooth it into a thin, even layer. Set aside. Reserve any remaining caramel sauce for a second tart or another use.
Make the ganache: In a small bowl, add the chocolate, butter, corn syrup, and salt. In a small pot set over high heat, add the heavy cream and bring it to a simmer. Immediately pour the cream it into the bowl, then tap the sides of the bowl gently to settle any chocolate or butter under the surface of the hot liquid. Set aside for 5 minutes to begin melting the ingredients, then use an immersion blender (or alternatively, a whisk or rubber spatula) to fully melt and emulsify the ganache, trying not to create too many air bubbles in the process. Once the mixture is shiny and smooth, slowly pour it over the set caramel inside the tart shell, filling it to the very top. Let the tart set at room temperature for at least 1 hour before serving. (Do not refrigerate or the ganache will harden.)
If serving with crème fraîche, thin a small amount by whisking it or by adding a little water if needed. Using a hot knife, cut the tart into wedges, wiping the knife with a damp towel between each cut. Transfer the wedges to dessert plates, sprinkle with flaky sea salt, and drizzle with the crème fraîche, if desired.