In a medium bowl, sift flour, cocoa and baking soda. In a large bowl or stand mixer, beat softened butter until smooth. Add in sugars, salt and vanilla. Scrape the sides of your bowl well. Add flour mix gradually. Do not overmix dough. Stir in chocolate pieces.
Form dough into one or two logs, wrap in plastic. Make sure to form your logs as wide around as you want your cookies to be. At first I made them pretty small around as that is what was I read, but then realised they'd make seriously small cookies. I pounded and reshaped the dough into a nice thicker log. Chill a minimum of 3 hours, or overnight as I did. Store dough in the fridge for up to 3 days or in the freezer for 2 months.
Using a sharp knife, make slices that are approximately 1/2" thick. Bake 12 minutes in a 325F° oven. Let cookies sit on baking sheet for several minutes prior to attempting to transfer any cookies to a cooling rack. Be extremely careful as these are very delicate. Slide the cookie on your spatula and slide off just as gentle. Whenever transferring the cookies from one location to another, I suggest using a spatula to prevent breakage.
Eat warm or store in an airtight container for up to several days.
Nutritional information is provided as a courtesy and is an estimate only. Nutrition information can vary for a variety of reasons. For the most accurate data, use your preferred nutrition calculator with the actual ingredients you used to make this recipe.
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