Prepare the Crust: In a medium bowl, combine flour, cocoa powder, and salt; set aside. Using a stand mixer fitted with the paddle attachment, or in a large bowl with a electric hand mixer, cream the butter and sugar until light and fluffy; mix in yolks and vanilla. Add in dry ingredients. Divide dough into 6 equal portions. Evenly press each portion into the bottoms and sides of 6 3.5-inch tartlet pan with removeable bottoms. Refrigerate tartlet shells for 30 minutes. Preheat oven to 350 degrees F, prick the tart shells all over with a fork. Bake in preheated oven until cooked through, about 13-15 minutes. Transfer to a rack and let cool.
Prepare the Caramel: In a medium saucepan over medium heat, whisk together sugar, corn syrup, salt, and 6 tbsp. water and bring to a boil. Cook, without stirring, until a candy thermometer inserted into the syrup reads 340°. Remove pan from heat and whisk in butter, cream, sour cream and vanilla (the mixture will bubble up) until smooth. Pour caramel into cooled tartlet shells and let cool slightly; refrigerate until firm, about 3-4 hours.
Prepare the Ganache: In a medium saucepan over medium heat, bring cream to a boil. Put chocolate into a medium bowl and pour in hot cream; let sit for 1 minute, then stir slowly with a rubber spatula until smooth. Spoon ganache evenly over tartlets and refrigerate until set, 3 or 4 hours. Sprinkle top with sea salt, if desired, and serve.
FOR THE CRUST
FOR THE CARAMEL
FOR THE GANACHE