1. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, cocoa, and baking soda in a small bowl, stirring well with a whisk.
2. Combine the brown sugar and next 5 ingredients (brown sugar-peppermint extract) in a medium bowl; beat with a mixer at high speed for 2 minutes. Beat in egg white. Add flour mixture; beat at low speed until well-blended.
3. Coat your hands lightly with cooking spray. Shape the dough into a 6 inch log. Wrap in plastic wrap; freeze 1 hour or until firm.
4. Preheat oven to 350 degrees.
5. Cut the log into 24 (1/4 inch) slices, and place 1 inch apart on baking sheets lightly coated with cooking spray. Bake at 350 for 10 minutes. Remove from pans; cool on wire racks.
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