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1
In the bowl of a stand mixer fitted with a paddle attachment, cream butter and confectioners' sugar together until combined, about 1 minute. Add the yolk, and vanilla and beat until light and fluffy, 3 to 4 minutes. Sift in the flour, cocoa, and salt. Mix on low speed, just until the dough comes together. For any dry spots, mix by hand to even them out. Shape the dough into a disk and wrap in plastic. Chill in the refrigerator for at least 1 hour and up to three days.
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2
Adjust oven rack to middle position and preheat the oven to 350u00b0F. On a lightly floured surface, roll out the disk of chilled dough until it's just shy of 1/4-inch thick. Roll the dough around the rolling pin, and carefully transfer to the tart pan. Press the dough into the pan, patching any holes with excess dough. Chill the shell for at least 1/2 hour.
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3
Trim the edges of the tart so they are flat and uniform. Line the tart shell with foil and fill with pie weights or dried beans. Bake for 20 minutes, rotating halfway through. Remove the weights and liner, bake for an additional five minutes. Allow the shell to cool completely.
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4
Place 4 ounces (1/2 cup) of water in the bottom of a large saucepan. Add the sugar and corn syrup and cook the mixture over medium-high heat. Wash down the sides of the pot using a pastry brush dipped in cool water. The mixture will begin to turn amber and smoke a bit, at which point, you should swirl the contents to even out the mixture. After about 10 minutes, you will have a dark amber caramel. Remove it from the heat, add the butter, cream, creme fraiche, and salt (be careful as you do, the caramel mixture will sputter), and whisk until the caramel is smooth. Pour the caramel out into the cooled shell. Let the tart sit for 1 hour, until the caramel is cool.
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5
Bring the cream to a boil and place the chopped chocolate in a large bowl. Pour the cream over the chocolate and allow it to sit for a few minutes to melt the chocolate. Stir the ganache with a rubber spatula until it is smooth. Pour it over the top of the caramel tart, tilt the tart around to distribute it evenly. Allow the chocolate to set at room temperature for at least 2 hours before serving (for a tart with runny caramel) or in the fridge for at least an hour (for firm, chewy caramel). Sprinkle with sea salt just before serving.
