1. Heat oven to 350 degrees F. Line a 13x9-inch baking pan with foil; grease foil. Microwave unsweetened chocolate and butter in large microwaveproof bowl on 100 percent power (High) 3 minutes; stir until smooth. Whisk in sugar, then eggs and vanilla. Stir in flour and salt. Fold in chocolate chips and walnuts. Pour into pan.
2. Bake 28 to 30 minutes or until center is just set. Cool. Remove brownies from pan; peel off foil. Cut into 2-inch squares.