Place steel blade in food processor bowl. Add the 2-1/4 cups flour, the 3/4 cup butter and salt. Cover; process with on/off turns until most of mixture resembles cornmeal, but with a few larger pieces. With processor running, quickly add 6 tablespoons water through the feed tube. Stop processor when all the water is added scrape down sides. Process with 2 on/off turns (mixture may not all be moistened and will resemble coarse crumbs). Remove the dough from the food processor bowl; shape into a ball kneading with hands as necessary. (Or, in a medium bowl combine flour and salt; cut in 3/4 cup butter using a pastry blender. Make a well in center; add 6 to 8 tablespoons ice water, one tablespoon at a time. Toss mixture with a fork to moisten after each addition.)
Use your hands to flatten the dough into a disk. Cover the disk with plastic wrap and refrigerate for 1 hour or until the dough is easy to handle; unwrap.
On a lightly floured surface, roll the dough from center to edges to form a circle 15 inches in diameter. Fold the pastry in quarters and transfer the pastry to a 10-inch springform pan or 10-inch round cake pan. Unfold the pastry into the pan, being careful not to stretch the pastry. Gently press the pastry onto side of pan two inches up the sides of the springform pan or trim the edges flush with the top of the cake pan.
Generously prick bottom and sides of pastry in pan with a fork. Line pastry with a double thickness of regular foil or one layer of heavy-duty foil. Bake in a 450 degree F oven for 8 minutes. Remove foil. Bake for 4 to 5 minutes more or until set and dry. Set aside. Reduce oven temperature to 350 degree F.
In a large mixing bowl, beat the eggs with an electric mixer on high speed about 5 minutes or until thick and light colored. Gradually add the sugar; mix well until mixture is light and fluffy. Add the 3 tablespoons melted butter and vanilla; mix well. Add the flour; mix just until combined, scraping the sides of the mixing bowl. Stir in walnuts and chocolate pieces.
Pour the filling into the partially baked pastry shell. Bake in a 350 degrees degree F oven for 45 minutes until golden brown. Cool 1-1/2 hours on a wire rack. Remove sides of springform pan, or turn the tart in cake pan out onto rack, then invert onto cake plate. Serve with ice cream. Top of cake will crack when cut. Makes 12 to 16 servings.