1Heat oven to 350°F. Grease bottom and sides of 13x9-inch pan with shortening or cooking spray. In 2-quart saucepan, heat brown sugar and 1/4 cup butter over low heat, stirring occasionally, until butter is melted; cool slightly. Stir in 2 tablespoons flour, 1 teaspoon vanilla and 1 egg until blended. Stir in walnuts; set aside.
2In 3-quart saucepan, melt 3/4 cup butter and the chocolate over low heat, stirring constantly. Cool slightly. Stir in granulated sugar, 1 cup flour, 1 teaspoon vanilla and 4 eggs. In pan, spread batter evenly. Spoon topping evenly over batter.
3Bake 45 minutes (do not overbake). Cool completely in pan on cooling rack. For brownies, cut into 8 rows by 4 rows.