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 an electric hand mixer, cream the butter and sugar until light and fluffy; mix in egg 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 pans with removable 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, heavy 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 heavy 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, 3-4 hours. If desired, sprinkle tart with vanilla fleur de sel and slice. Serve cold.
