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Mix the flour, baking soda, cornstarch and salt in a large bowl. Set aside.
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Melt the butter in the microwave.
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In a medium bowl, whisk the melted butter, brown sugar, and white sugar together until fully incorporated.
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Whisk in the egg and then the egg yolk into the butter mixture.
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Finish the wet ingredients by whisking in the vanilla.
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Pour the wet ingredients into the dry ingredients and mix together with a rubber spatula. The dough will be very soft, yet thick.
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Next, fold in the chocolate chunks.
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Cover the dough and chill for at least 2 hours - longer is better - overnight is great . Chilling is the key to making this recipe work.
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Take the dough out of the refrigerator and allow to slightly soften at room temperature for 10 minutes.
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Preheat the oven to 325F degrees. Line the baking sheet with parchment paper or silicone baking mats. Set aside.
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Using a small ice cream scoop, scoop out the dough and roll into a ball. (You want the dough to be taller than wide).
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Bake the cookies for 11-12 minutes.
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Allow to cool.
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Serve and enjoy!