1 1/4 cups all-purpose flour
1/3 cup Dutch Process Cocoa
1/2 teaspoon baking soda
1/2 cup plus 3 tablespoons unsalted butter, room temperature
2/3 cup packed light brown sugar
1/4 cup granulated sugar
1/2 teaspoon salt
1 teaspoon pure vanilla extract
Sift the flour, cocoa, and baking soda together; set aside.
Put the butter in the bowl of a mixer and beat on medium speed until the butter is soft and creamy. Add brown sugar,granulated sugar, salt, and vanilla extract; beat for another 1 to 2 minutes.
Reduce the mixer speed to low and add the sifted dry ingredients. Mix only until the dry ingredients are incorporated (the dough will look crumbly. For the best texture, don't work the dough too much once the flour is added). Toss in the chocolate pieces and mix only to incorporate.
Turn the dough out onto a smooth work surface, divide it in half and, working with one half at a time, shape the dough into a log that is 1 inches in diameter. Wrap the logs in plastic wrap and refrigerate them for at least 1 hour.
Preheat oven to 350 degrees and place on center rack in the oven. Working with a sharp thin-bladed knife, slice rounds that are 1/2-inch thick. Place the cookies on the cookie sheets leaving about 1 inch space between each cookie.
Bake only one sheet of cookies at a time and bake each sheet for 12 minutes. The cookies will not look done, nor will they be firm, but they will finish cooking while cooling. Remove from over and let the cookies cool on the baking sheet, until they are only just warm or until they reach room temperature, then place on plate. Makes about 36 cookies.
NOTE: This dough can be made ahead and be either chilled or frozen. Wrapped them in an airtight container. The logs can be refrigerated for up to 3 days or frozen for 1 month. If the dough is frozen, don't defrost it before baking—just slice the logs into cookies and bake the cookies 1 minute longer. Packed airtight, baked cookies will keep at room temperature for up to 3 days; they can be frozen for up to 1 month.