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Adjust oven rack to middle position and heat oven to 350 degrees. Make foil sling for 13 by 9-inch baking pan by folding 2 long sheets of aluminum foil; first sheet should be 13 inches wide and second sheet should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Lightly spray with vegetable oil spray.
Whisk flour, salt, and baking soda together in a bowl.
Whisk sugar, melted butter, egg and whites, vanilla, and vinegar together in another large bowl until smooth.
Stir in flour mixture and chocolate into the bowl with wet ingredients with rubber spatula until just combined.
Transfer batter to prepared pan and smooth top. Bake until toothpick inserted in center comes out with few moist crumbs attached, 17 to 20 minutes, rotating pan halfway through baking. Let blondies cool completely in pan on wire rack, about 2 hours. Using foil overhang, lift blondies out of pan. Cut into 16 pieces and serve.