Preheat oven to 325 degrees. Grease a 9 inch square baking pan. In a small bowl, mix together flour and baking soda. In a small saucepan, combine sugar, butter and water. Bring to a boil over medium heat; remove immediately from heat. Stir in 1 cup of chocolate chips and vanilla until chocolate is melted and mixture is smooth. Transfer mixture to a medium bowl. Cool completely. Stir in eggs, 1 at a time, beating well after each addition. Gradually stir in flour mixture until smooth. Stir remaining chocolate chips and nuts into batter. Pour batter into prepared pan. Bake until a toothpick inserted in center comes out clean, 30-35 minutes. Transfer pan to a wire rack to cool completely. Cut into squares.
For light, moist brownies, cool chocolate mixture completely before combining it with other ingredients. Test for doneness 2-3 minutes before suggested baking time.