In a large bowl, combine the flour, confectioners' sugar and salt; cut in butter until crumbly. Gradually add water and vinegar, tossing with a fork until dough forms a ball. Wrap in plastic wrap. Refrigerate for 30 minutes or until easy to handle.
Roll out pastry to fit a 9-in. pie plate. Transfer pastry to pie plate. Trim pastry to 1/2 in. beyond edge of pie plate; flute edges.
In a large bowl, whisk the filling ingredients until blended. Pour into pastry.
In a small bowl, combine brown sugar and flour. Cut in butter until crumbly. Stir in hazelnuts and liqueur if desired; sprinkle over filling. Cover edges loosely with foil. Bake at 375° for 50-55 minutes or until a knife inserted near the center comes out clean.
Cool on a wire rack. Refrigerate leftovers. Yield: 8 servings.