1. Heat oven to 350 degrees F. Coat 13 x 9 x 2-inch baking pan with nonstick cooking spray.
2. In large bowl, whisk together 2 cups of the flour, baking soda and salt.
3. In another large bowl, beat butter and sugar until smooth and fluffy, about 2 minutes. Beat in egg and vanilla. On low, beat in flour mixture just until combined. Fold in 1 cup of the nuts.
4. Reserve 1/2 cup dough. With floured fingers, press remaining dough evenly over bottom of prepared pan. Spread melted chocolate over dough. With hands, mix remaining nuts and the 3 tablespoons flour into reserved 1/2 cup dough; crumble nut mixture evenly over chocolate.
5. Bake at 350 degrees F for 20 minutes or until golden. Let cool before cutting.