In a medium bowl, whisk flour, cornstarch, baking soda and salt. Set aside.
In a separate large bowl, or the bowl of your stand mixer fitted with the paddle attachment, beat butter and sugars until light and fluffy. Add egg and vanilla extract. Add flour mixture, mixing until completely combined. Do not over-mix. Fold in chocolate chips.
Transfer dough to a 10-inch ovenproof skillet. Press to flatten, covering bottom of pan. Bake until edges are brown and top is golden, about 30 minutes. Don't overbake; it will continue to cook a few minutes out of the oven.
Transfer to a wire rack to cool for 15 to 20 minutes. Cut into 8 wedges. Serve warm.
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