For the crust, whisk to combine the flour, cocoa powder and salt; set aside.
In another bowl, cream together the butter and sugar. Add in the egg yolks and vanilla and mix until combined. Little by little, add the dry ingredients until incorporated. The dough may look crumbly, but it should come together easily when you pinch the dough between two fingers.
Press the dough out into a tart pan. Prick all over with a fork and put into the freezer for 10 minutes. Bake chilled crust for 15 minutes, then cool completely on a wire rack.
For the caramel, place the sugar, corn syrup and salt in a heavy bottomed saucepan over medium high heat. Stir to combine, but onces the mixture begins to bubble stop stirring for the remainder of the cooking process. The sugar will first turn clear, then it will noticeably start bubbling and eventually take on color. Continue to cook, without stirring, until a candy thermometer inserted into the syrup reads 340°, stir in the butter. Remove the pan from the heat and carefully (watch the sputtering) add the heavy cream and vanilla. Pour the caramel into the cooled crust and refrigerate for about 3-4 hours or until set.
To make the chocolate ganache, heat the heavy cream to a simmer, then pour it over the chocolate. Let it sit for 60 seconds, then stir until smooth. Pour the chocolate ganache over the caramel tart and refrigerate for a few hours until set.
To serve, cut into slices and garnish with sea salt.