hazelnut crust

1. Finely grind hazelnuts in a coffee or spice grinder, then add the sugar and grind again. Transfer to the bowl of a mixer fitted with the paddle attachment, add the salt and butter, and cream the mixture until smooth.
2. Gradually add the flour, mixing until the dough comes together and adding ice water as necessary. Remove the dough from the mixer, form it into a disc, and wrap it in plastic wrap. Chill in the refrigerator for at least 1 hour before rolling (the dough can be refrigerated overnight).
3. Butter and flour a 10 1/2-inch to 11-inch tart pan with a removable bottom.
4. Flour a pastry board or other work surface. Roll the dough into a 14-inch circle about 1/4-inch thick, flouring the dough and board as necessary to keep the dough from sticking. Roll up the dough around the rolling pin and unroll it into the tart pan. Gently lift the edges to ease the dough into the corners of the pan, then push down gently to be certain the dough around the sides is 1/4-inch thick. Cut away excess dough. If there are any thin spots in the sides of the tart shell, pat a bit of the dough scraps over them to create an even thickness.
5. Prick the bottom of the crust with a fork and refrigerate for at least 15 minutes.
6. Preheat over to 375°F.
7. Line the tart shell with parchment paper or aluminum foil and fill with dry beans or pie weights. (Do not push down on the beans or they may damage the dough). Bake for about 20 minutes, or until the crust is a light golden brown. Do not rush this process- if the weights are removed too early, the sides of the crust will shrink. Remove the parchment and beans and bake the crust for another 15 to 20 minutes, or until it is a rich golden brown and the bottom is thoroughly cooked. Place on a cooling rack and let cool completely.

dark chocolate mousse

1. Combine the chocolate, butter, and espresso in the top of a double boiler over hot, but not simmering, water, stirring frequently until smooth. Remove from the heat and let cool until the chocolate is just slightly warmer than body temperature. To test, dab some chocolate on your bottom lip. It should feel warm. If it is too cool, the mixture will seize when the other ingredients are added.
2. Meanwhile, whip the cream to soft peaks, then refrigerate.
3. Once the melted chocolate has cooled slightly, whip the egg whites in a medium bowl until they are foamy and beginning to hold a shape. Sprinkle in the sugar and beat until soft peaks form.
4. When the chocolate has reached the proper temperature, stir in the yolks. Gently stir in about one-third of the whipped cream. Fold in half of the egg whites until just incorporated, then fold in the remaining whites, and finally the remaining whipped cream.

assembly

1. Place the mousse in a pastry bag fitted with a large plain tip and pipe it into the tart shell, or just spread it evenly in the shell with a spatula. Smooth the top with a small offset spatula. Refrigerate, covered, for 8 hours to allow the mousse to set completely.
2. When ready to serve, dust the top of the tart with a light sprinkling of cocoa and sprinkle with the chopped hazelnuts. Enjoy!