Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper and spray with nonstick spray. Set aside.
In a medium bowl, whisk the flour, baking soda, corn starch, and salt together.
2½ cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon corn starch, ½ teaspoon kosher salt
To the bowl of a stand mixer fitted with the paddle attachment, add the butter, brown sugar, and granulated sugar. Beat until light and fluffy, about 1 -2 minutes. Scrape down the sides of the bowl as needed.
1 cup unsalted butter, 1 cup brown sugar, ½ cup granulated sugar
Add the eggs and vanilla and beat until well combined, about 1 minute.
2 large eggs, 2 teaspoons pure vanilla extract
Mix the dry ingredients into the wet ingredients. Beat just until mixed and the dough starts sticking together. Scrape down the sides of the bowl as needed.
Gently fold in 1 cup of the chocolate chips.
1½ cups semisweet chocolate chips
Transfer the cookie dough to the prepared pan. Use a rubber spatula or the back of a spoon to spread the cookie dough into an even layer.
Sprinkle the remaining ½ cup of chocolate chips over the top of the cookie dough.
Bake for 15 minutes.
After 15 minutes, loosely cover the baking pan with aluminum foil. Bake for another 35 minutes, or until the edges are set and the middle is slightly browned. A toothpick inserted in center should come out clean.
Remove the pan from the oven and leave the cookie bars in the baking pan until completely cool.
Once cool, remove the cookie bars from the baking pan and slice into 16 squares.