Step 1: Preparing the Crust

1. In a medium bowl, whisk together the flour, cocoa powder, and salt; set aside.
2. Using a stand mixer fitted with the paddle attachment (or a hand mixer), cream the butter and sugar together until light and fluffy.
3. Add the egg yolks and vanilla extract, mixing until combined.
4. Slowly incorporate the dry ingredients, mixing just until a dough forms.
5. Divide the dough into six equal portions and press each portion into the bottoms and sides of six 3.5-inch tartlet pans with removable bottoms.
6. Refrigerate the tartlet shells for 30 minutes to prevent shrinkage during baking.
7. Preheat your oven to 350°F (175°C). Prick the tart shells all over with a fork to prevent air pockets.
8. Bake for 13-15 minutes, until set. Transfer to a wire rack and let cool completely.

Step 2: Making the Caramel Filling

1. In a medium saucepan over medium heat, whisk together sugar, corn syrup, salt, and water.
2. Bring to a boil and cook without stirring until a candy thermometer reads 340°F (170°C). The mixture should turn a deep amber color.
3. Remove from heat and carefully whisk in butter, heavy cream, sour cream, and vanilla extract. Be cautious, as the mixture will bubble up rapidly.
4. Stir until smooth, then pour the caramel into the cooled tart shells.
5. Let the tarts cool slightly, then refrigerate for 3-4 hours, or until the caramel is firm.

Step 3: Preparing the Chocolate Ganache

1. In a medium saucepan over medium heat, bring heavy cream to a gentle boil.
2. Place chopped chocolate in a medium bowl and pour the hot cream over it. Let sit for 1 minute to allow the chocolate to soften.
3. Slowly stir with a rubber spatula until the ganache is smooth and glossy.
4. Spoon the ganache evenly over the chilled caramel tarts.
5. Refrigerate again for 3-4 hours until the chocolate is set.
6. If desired, sprinkle with Vanilla Fleur de Sel or sea salt before serving.