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Place the flour, cocoa, sugar, baking soda and salt into a large (16+ cups) food processor. Process for a few seconds to mix. Add the butter in chunks. Run the processor for another 5-10 seconds. Use a spatula to scrape the bottom and sides if needed. With the processor running, add the milk and vanilla until the dough clumps around the processor blades.
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Scrape the dough onto a clean work surface and knead briefly to finish mixing. Knead or mix in any of the options listed above.
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If you are making slice-and-bakes, roll the dough into logs and chill for at least 3 hours.
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When you are ready to bake, either right away or after chilling the logs, Preheat the oven at 350 F.
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Bake at 350 F until done--the thinner the cookie, the longer they will take to bake. I started checking at 8 minutes for individual cookies and 20 minutes for bars. I took them out when the tops got a little dull but were still soft. Play around and experiment!