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1
Line a 12-by-17-inch baking sheet with parchment paper.
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2
Into a medium bowl, sift together the flour, cocoa powder, baking soda and salt.
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3
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until creamy.
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4
Add 2/3 cup of the sugar and beat until light in color and texture, about 3 minutes.
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5
Add the egg and vanilla extract and beat until the egg is fully absorbed and the mixture lightens in color, 1 minute more.
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6
Reduce the speed to low and gradually stir in the flour mixture just until combined (avoid over-mixing or the cookies will be dry!).
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7
Stir in the chocolate.
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8
Use a 1 tablespoon measuring spoon to portion the dough onto the prepared baking sheets- 12 cookies per sheet, evenly spaced (the cookies won't spread too much during baking).
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9
Freeze until the dough is solid, about 30 minutes.
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10
Meanwhile, position a rack to the center of the oven and preheat the oven to 350 degrees F.
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11
Bake the cookies just until they are slightly puffed and the edges can be lifted off the parchment with a fingertip, but the cookies look somewhat underdone and soft towards the center, 10 minutes, being careful not to over-bake.
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12
Cool the cookies completely on the baking sheet set over a wire rack.
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13
Store in an airtight container at room temperature for up to 5 days.
