To prepare the tart dough, in the bowl of an electric mixer fitted with the paddle attachment, cream the butter and confectioners' sugar until combined, about 1 minute. Add the cocoa and beat until incorporated. Add the egg yolk and vanilla and beat until smooth. Sift in the flour 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 oven to 325 degrees F.
On a lightly floured surface, roll the tart dough 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 dozen 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 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. Remove cooled shells from tart pans and transfer to a plate.
To prepare the filling, place the water in a 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 amber 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 20 minutes.
To make the glaze, in a saucepan, bring the cream to a boil. Place the chocolate in a bowl. Pour the hot cream over the chocolate, add a pinch of sea salt, and let sit for 2 minutes, then whisk until smooth. Pour some of the glaze over each of the tarts while it is still warm. Let the glaze set at room temperature for at least 2 hours before serving.
Recipe courtesy of Claudia Fleming