Combine the flour, cocoa, soda, and salt in a medium bowl. Mix thoroughly with a whisk. Set aside. Combine the sugars in a small bowl and mix well with fingers pressing out any lumps. (Process in a food processor if lumps are stubborn).
In a medium mixing bowl, beat butter and margarine until creamy. Add sugar mixture, cinnamon, peppers, and vanilla. Beat on high speed for about one minute. Beat in egg white. Stop the mixer. Add the flour mixture. Beat on low speed just until incorporated. Gather the dough together with your hands and form it into a neat 9 to 10-inch log. Wrap in waxed paper. Fold or twist ends of paper without pinching or flatting the log. Chill at least 45 minutes, or until needed.
Place oven racks in the upper and lower third of the oven preheat to 350 degrees. Line cookie sheet with parchment paper or aluminum foil.
Use a sharp knife to slice rounds of chilled dough a scant 1/4-inch thick. Place 1-inch apart on prepared baking sheets. Bake 12 to 14 minutes. Rotate baking sheets from top to bottom and front to back about halfway through. Cookies will puff and crackle on top, and then begin to settle down slightly when done. Use a metal spatula to transfer cookies to a wire rack to cool. Allow cookies to cool completely before storing or stacking. Store in an airtight container up to two weeks, or freeze up to 2 months.