1. Sift flour, cocoa, and baking soda together.
2. In large bowl of stand mixer with paddle attatchment, beat butter, sugar, and salt together on medium speed until smooth, about 3 minutes, scraping down bowl as needed.
3. On low speed, blend in vanilla and egg white, beating 2 minutes.
4. Turn mixer off, add half flour-cocoa mixture and pulse at first, then mix on low to incorporate. Scrape bowl and repeat with remaining mixture.
5. Transfer dough to work surpface, divide in half, and shape into disks. One at a time, roll out disks between parchment paper to 1/8 inch thick. Slide onto baking sheet and freeze 1 hour.
6. Preheat oven to 350F. Using 2-inch scalloped cookie cutter, cut dough and place on parchment-lined baking sheets. Reserve scraps to combine and re-roll out.
7. Bake 15 to 17 minutes, rotating pans halfway.
8. Let cookies cool 2 minutes on baking sheets before transferring to cooling rack.
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