To prepare the tart dough, in the bowl of an electric mixer fitted with the paddle attachment, cream the butter and confectioners' sugar until just combined, about 1 minute. Add the egg yolk and vanilla and beat until smooth. Sift in the flour and cocoa powder and beat on low speed until just combined. Scrape the 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 the oven to 325 degrees F. On a lightly floured surface, roll the 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 the 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 the foil and weights and bake for 5 to 10 minutes longer, or until the pastry looks dry and set. Transfer to a wire rack to cool. (The tart shells can be made 8 hours ahead.)

To prepare the filling, place 1/2 cup water in large saucepan. Add the sugar and corn syrup and cook the mixture over medium-high heat, swirling the pan occasionally, until you have a dark caramel, about 10 minutes. Carefully whisk in the butter, cream, and creme fraiche (the mixture will hiss and bubble up, so stand back), whisking until smooth. (The caramel can be made up to 5 days ahead and refrigerated.) Divide the caramel among the tart shells while still warm (or reheat the caramel in the microwave or over low heat until it is pourable) and let sit until the caramel is set, at least 45 minutes.

To make the ganache glaze, in a saucepan, bring the cream to a boil. Place the chocolate in a bowl. Pour the hot cream over the chocolate and let sit for 2 minutes, then whisk until smooth. Pour some of the glaze over each of the tarts while still warm. Let the glaze set at room temperature for at least 2 hours before serving. Sprinkle with salt, if desired, just before serving.

Variation: For a large tart, line a 10-inch tart pan with the pastry dough, then prick, weight, and bake as directed, adding 5 to 10 minutes to the baking time. When the tart shell is cool, spoon the warm caramel to set before pouring the warm ganache onto the tart.