Chocolate Wafers: In a bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In the bowl of your electric mixer (or with a hand mixer), cream the butter and margarine until well blended. Add the sugars and vanilla extract and beat on high speed for about one minute. Scrape down the sides of the bowl. Beat in the egg white. Add the flour mixture and beat just until incorporated.

Place the dough on your work surface and, using your hands, form the dough into a log shape that is about 9 inches (23 cm) long. Carefully wap the dough in aluminum foil, parchment paper or wax paper and fold or twist the ends. Try not to flatten the log. Refrigerate for at least one hour or overnight.

Preheat the oven to 350 degrees F (177 degrees C) and place the oven rack in the center of the oven. Line two baking sheets with parchment paper.

Using a sharp knife, slice into 1/4 inch thick wafers. Place on the baking sheet spacing about 1 inch (2.5 cm) apart. Bake for approximately 10-12 minutes or until the the cookies puff and the tops of the cookies have crackles (ripples). Remove from oven and let cookies cool on baking sheet for about 5 minutes before removing to a wire rack to cool completely. Can be stored in an airtight container, at room temperature, for about 10 days.

Makes about 36 cookies.