Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup, and level with a knife. Combine the flour, baking soda, and salt in a small bowl, stirring well with a whisk. Set aside.
Combine brown sugar, vanilla, and egg whites in a large bowl; beat with a mixer at high speed 1 minute. Add flour mixture, beating just until combined. Stir in pecans and chocolate chips.
Spread batter into an 8-inch square baking pan coated with cooking spray. Bake bars at 350° for 18 minutes or until golden and crusty on top. Cool in pan 10 minutes. Sprinkle with powdered sugar.