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1
Dough: Use an electric mixer with paddle attachment to cream butter and confectioners' sugar until combined, about 1 minute. Add egg yolk and vanilla, and beat until smooth. Sift in flour and cocoa powder, and beat on low until just combined. Scrape the dough onto a sheet of plastic wrap, and form it into a disk; wrap well. Chill until firm, at least 1 hour and up to 3 days.
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2
Preheat oven to 325u00b0 F. On a lightly floured surface, roll the tart dough into a large circle 3/16 inch thick. Transfer to a 10-inch fluted tart pan with a removable bottom and press into pan. If it falls apart just push it back together in the pan. Refridgerate for 30 minutes.
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3
Prick the shell all over with a fork. Line with parchment paper filled with pie weights or dried beans and blind bake for 15 minutes. Remove the parchment and weights, and bake until the pastry looks dry and set, 5 to 10 minutes. Transfer pan to a wire rack to cool. (Can be made 8 hrs ahead.)
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4
Filling: Place 1/2 cup water in a large saucepan. Add sugar and corn syrup, and cook mixture over medium-high heat, swirling the pan occasionally, until it becomes a dark-amber caramel (365F), about 10 minutes. Remove pan from heat and slowly (mixture will bubble)add heavy cream followed by butter and creme fraiche. Stir until smooth. (Caramel can be made up to 5 days ahead and refrigerated in a covered container.) Pour the caramel into the cooled tart shell and allow to set, first at room temperature and then in the fridge.
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5
Ganache glaze: Place the chocolate in a heatproof bowl. In a small saucepan, bring cream to a boil. Pour the hot cream over the chocolate, and let stand for 2 minutes, then stir with a rubber spatula until smooth. Pour the ganache over the tart. Refrigerate until set.
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6
Remove the tart from the refridgerator 5-10 minutes before you are ready to serve it. Cut the tart into slices and sprinkle each with Fleur de Sel.
