Chocolate tart dough: In bowl of electric mixer fitted with paddle attachment, 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 speed until just combined. Scrape dough onto a sheet of plastic wrap and form it into a disk. Wrap and chill until firm, about 1 hour, or up to 3 days.
Preheat oven to 325 degrees F. On lightly floured surface, roll tart dough to an 18 by 12-inch rectangle, 3/16-inch thick. Using a 2 1/2-inch round cutter, cut out 24 rounds of dough and press them into mini muffin tins or 2-inch tart pans, trimming away any excess dough. Prick the dough all over with a fork. Chill tart shells for 20 minutes.
Line the tart shells with foil and fill with dried beans, rice, or pie weights. Bake for 15 minutes. Remove foil and weights and bake for 5 to 10 minutes longer, or until pastry looks dry and set. Transfer to a wire rack to cool. (The tart shells can be made 8 hours ahead.)
Caramel 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 you have a dark amber caramel, about 10 minutes. Carefully whisk in butter, cream, and creme fraiche (mixture will hiss and bubble up), whisking until smooth. (Caramel can be made up to 5 days ahead and refrigerated, then warmed over low heat or microwaved before pouring into tart shells.)
Divide caramel while still warm among cooled tart shells and let sit until caramel is set, at least 45 minutes.
Chocolate ganache glaze: In saucepan, bring cream to a boil. Place chopped chocolate in a bowl. Pour hot cream over chocolate and let sit for 2 minutes, then whisk until smooth. Pour some of the still-warm glaze over each tart. Let glaze set at room temperature for at least 2 hours before serving. Sprinkle with salt, if desired, just before serving.
Recipe courtesy of Claudia Fleming, "The Last Course: The Desserts of Gramercy Tavern"