Heat oven to 350°F. Grease 13x9x2-inch baking pan.
Place butter in large microwave-safe bowl. Microwave at MEDIUM (50%) 2 to 2-1/2 minutes or until melted. Stir in sugar and vanilla. Add eggs, one at a time, beating well with spoon after each addition. Add cocoa; beat until well blended. Add flour, baking powder and salt; beat well. Stir in nuts, if desired. Pour batter into prepared pan.
3. 3 Bake 30 to 35 minutes or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack. Frost if desired. Cut into bars.