1. Preheat oven to 400ºF.
2. To prepare crust: Coat a 9-inch tart pan with cooking spray. Combine whole-wheat pastry flour, 1/4 cup all-purpose flour, hazelnuts, 1/4 cup sugar and salt in a food processor; process until the nuts are finely ground. Add butter one piece at a time, pulsing once or twice after each addition, until incorporated. Add oil and ice water and pulse just until incorporated. Turn the dough out into the prepared pan (it will be crumbly), spread evenly and press firmly into the bottom and all the way up the sides to form a crust.
3. Bake the crust until set and the edges are beginning to brown, about 15 minutes. Let cool on a wire rack.
4. To prepare filling: Sprinkle gelatin over water in a small bowl; let stand, stirring once or twice, while you prepare the rest of the filling.
5. Heat milk in a medium saucepan over medium heat until steaming (but not boiling); remove from the heat to cool slightly.
6. Whisk egg yolks, 2 1/2 tablespoons sugar and 1 tablespoon flour in a medium bowl until combined. Gradually whisk in 1/2 cup of the hot milk. Whisk the egg yolk mixture into the pan with the remaining hot milk. Return to the heat and cook, stirring constantly, until the mixture thickens enough to coat the back of a spoon (do not boil), about 1 minute. Remove from the heat; whisk in chocolate until completely melted. Whisk in the softened gelatin and coffee liqueur (if using) until smooth.
7. Beat reconstituted egg whites and cream of tartar in a large bowl with an electric mixer on low speed until frothy. Increase speed to high and beat until soft peaks form. Gradually add the remaining 1/4 cup sugar and beat until stiff peaks form, 3 to 5 minutes. Gently fold the chocolate custard into the egg whites until blended. Spoon the filling into the crust; smooth the top with the back of a spoon and chill, uncovered, until set, about 1 hour.