Preheat oven to 350F (175C) and prepare a 9x9" (23x23cm) pan (see note) by lining with parchment paper (or lightly grease and flour).
Combine melted (cooled) butter and sugars in a large bowl, stir well.
1 cup unsalted butter, 1 cup light brown sugar, ¾ cup granulated sugar
Add eggs, egg yolk, and vanilla extract and stir until well-combined.
2 large eggs + 1 large egg yolk, 1 ½ teaspoons vanilla extract
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
2 ¼ cups all-purpose flour, 2 teaspoons cornstarch, ½ teaspoon baking powder, 1 teaspoon salt
Gradually stir dry ingredients into wet until completely combined. Stir in chocolate chips.
2 cups semisweet chocolate chips
Spread batter evenly into prepared pan. Bake on 350F (175C) for 35 minutes or until edges just begin to turn golden brown.
Allow to cool before cutting and serving (don't cool too long, though -- these are amazing when served still slightly warm!)
Video
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Notes
This recipe can also be baked in a 9x13 pan for about 24-30 minutes. Note that recipe times given are for metal pans, if you use a ceramic or glass pan the cookie bars will likely take longer to bake through, so always use the toothpick test to check for doneness.
Storing
Store in an airtight container at room temperature for up to 5 days. These cookie bars may also be frozen.