In a microwave or double boiler, melt chocolate and butter; cool for 10 minutes. Add sugar; mix well. Stir in eggs and vanilla. Add flour; mix well. Stir in the walnuts. Line a 13x9x2-inch baking pan with foil and grease the foil. Pour batter into pan. Bake at 350 ° for 25-30 minutes or until a toothpick inserted in the center comes out with moist crumbs (do not overbake). Cool completely. For topping, melt chocolate chips, water and butter in microwave or double boiler; stir until smooth. Cool to room temperature. Fold in whipped cream. Spread over brownies. Chill before cutting. Store leftovers in the refrigerator.