- Adjust oven rack to middle position; heat oven to 325 degrees. Line 9x13 backing dish with foil and spray foil-lined pan with nonstick cooking spray.
- Whisk to combine flour, salt, and baking powder in medium bowl; set aside.
- In microwave, heat chocolate and butter in large microwave-safe bowl on high for 45 seconds, then stir and heat for 30 seconds more. Stir again, and, if necessary, repeat in 15-second increments; do not let chocolate burn.
- When chocolate mixture is completely smooth, gradually whisk in sugar. Add eggs one at a time, whisking after each addition until thoroughly combined. Whisk in vanilla. Add flour mixture in three additions, folding with rubber spatula until batter is completely smooth and homogeneous.
- Transfer batter to prepared pan; using spatula, spread batter into corners of pan and smooth surface. Bake until toothpick or wooden skewer inserted into center of brownies comes out with few moist crumbs attached, 30 to 35 minutes. Cool on wire rack to room temperature.
Source: Cook's Illustrated, March/April 2004