Super-Chewy Chocolate Chip Cookie Bars

Ingredients

• 2 3/4 cups all-purpose flour
• 1 teaspoon cornstarch
• 1 teaspoon baking soda
• 3/4 teaspoon salt
• 3/4 cup unsalted butter
• 2 cups packed light brown sugar
• 2 large eggs plus 1 egg yolk
• 2 teaspoons pure vanilla extract
• 1 1/2 cups semi-sweet chocolate chips
• 1 cup chopped walnuts