1/3 cup each of 1/2-inch squares of bittersweet chocolate, milk chocolate and white chocolate (about 5 ounces total)
In a food processor, pulse together the flour, sugar, cocoa, salt and baking soda. Add the butter and pulse until the mixture resembles coarse meal. Drizzle in the ice water and pulse until the dough can be gathered into a ball. On a work surface, roll the dough into an 8-inch log. Wrap the log in plastic wrap and refrigerate for at least 1 hour or overnight.
Butter eight 2/3-cup ramekins (3 1/2 by 1 1/2 inches). Cut the log of dough into 8 equal pieces and roll them into balls. Pat each ball into a 4-inch round and line the ramekins with the dough, pressing it evenly over the base and up the sides to the top of the ramekins. Refrigerate until firm.
Preheat the oven to 350°. Spread the hazelnuts on a baking sheet and bake for 12 minutes, or until the skins blister. Wrap the hazelnuts in a towel and rub them together vigorously to remove most of the skins.
In a large bowl, whisk the eggs lightly, then whisk in the corn syrup, sugar, butter and vanilla until the sugar dissolves, about 1 minute. Stir in the hazelnuts and the chocolate. Set the ramekins on a large baking sheet and spoon the filling into the shells.
Bake for 30 minutes, then cover lightly with foil and bake for 15 minutes longer, or until the hazelnuts are browned and the filling is bubbling in the center. Let cool. Serve in the ramekins.