Heat oven to 350˚. Combine flour, cocoa powder, and salt in a medium bowl and set aside. Using a handheld mixer, cream the butter and sugar in a large bowl until mixture is pale and fluffy; mix in yolks and vanilla. Mix in dry ingredients. Transfer dough to a 9″ fluted tart pan with a removable bottom and press dough evenly into bottom and sides of pan. Refrigerate for 30 minutes. Prick the tart shell all over with a fork and bake until cooked through, about 20 minutes. Transfer to a rack and let cool.

In a 1-qt. saucepan, 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, and crème fraîche (be careful – the mixture will bubble up) until smooth. Pour caramel into cooled tart shell and let cool slightly; refrigerate until firm, 4–5 hours.

Bring cream to a boil in a 1-qt. saucepan over medium heat. 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. Pour ganache evenly over tart and refrigerate until set, 4–5 hours. Sprinkle tart with sea salt immediately before serving, slice, and serve chilled.