Preheat the oven to 350F.
In the bowl of a stand mixer with a paddle, cream together butter and both sugars until fluffy and light colored. Blend in the egg and vanilla.
Mix together the flour, cornstarch, baking soda and salt in a small bowl. Mix the flour mixture into the butter-sugar mixture by hand. Stir in chocolate chunks. The batter is very stiff.
Using a standard-sized cookie scoop or tablespoon, drop dough onto a parchment or Silpat lined baking sheet. Bake for 8-10 minutes, until just barely golden brown around the edges. Do not allow the cookies to brown on top, just the very edges.
Let cool, on the sheet, on a wire rack for five minutes to set. Remove from the baking sheet and let cool completely on the wire rack,
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