2 cups chocolate chips or chunks (semi or bittersweet)
1. Heat oven to 325 degrees. Adjust oven rack to lower-middle position. Cut 18-inch length foil and fold lengthwise to 8-inch width. Fit foil into length of 13 by 9-inch baking pan, pushing it into corners and up sides of pan; allow excess to overhang pan edges. Cut 14-inch length foil and fit into width of baking pan in same manner, perpendicular to first sheet (if using extra-wide foil, fold second sheet lengthwise to 12-inch width). Spray foil-lined pan with nonstick cooking spray.
2. Mix flour, salt, and baking soda together in medium bowl; set aside.
3. Whisk melted butter and sugars in medium large until combined. Add egg, egg yolk, and vanilla and mix well. Using rubber spatula, fold dry ingredients into egg mixture until just combined; do not overmix. Fold in chips and turn batter into prepared pan, smoothing top with spatula.
4. Bake until top is light golden brown, slightly firm to the touch, and edges start pulling away from sides of pan, 27 to 30 minutes. Cool on wire rack to room temperature. Remove bars from pan by lifting foil overhang and transfer to cutting board. Cut into 2-inch squares and serve.
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