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Add the flour to a medium bowl. Sift in the cocoa powder. Whisk to thoroughly combine.
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In a large bowl using a hand mixer on high speed, cream together the sugar and butter.
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Add the salt, egg, water, and vanilla to the creamed mixture. Beat on medium speed until smooth and thoroughly combined.
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Add the dry ingredients and beat until just combined. Do not over mix. It will be a stiff dough.
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Divide the dough in half. On a worksurface lightly dusted with cocoa powder, shape and roll each half into a log about 2 inches in diameter.
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Wrap each log in parchment paper and freeze for 8 hours or overnight. (Dough can be kept frozen for up to 2 weeks).
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Preheat the oven to 325 degrees F. Line baking sheets with a non-stick liner or parchment paper.
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Unwrap 1 log and cut into 1/8-inch thick rounds. Space 1/2-inch apart on the baking sheets.
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Place the baking sheets with the dough rounds in the refrigerator for 30 minutes.
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Re-wrap and place dough in freezer between batches.
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Bake the cookies for 16 minutes. Check the cookies at 15 minutes for the first sign of the edges darkening. These cookies burn or scorch easily.
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Remove the cookies immediately from the baking sheet and cool completely on a wire rack.
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Check the bottom of the cookies after each batch. If too dark or burned reduce the amount of baking time.
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Completely cool the baking sheets before using again for the next batch of cookies. Refrigerate them if necessary.