adapted from King Arthur Flour

2 1/4 cups powdered sugar

1/4 teaspoon salt

1 teaspoon espresso powder

1 cup unsweetened cocoa powder

3 egg whites

2 teaspoons vanilla extract

In a bowl, whisk together the sugar, salt, espresso powder and cocoa powder.  Put the egg whites and vanilla into your mixing bowl.  On low speed, add the dry ingredients to the wet ingredients.  The batter will seem dry at first, but after about 2-3 minutes of mixing on low speed it will become very shiny.  Cover the bowl and refrigerate for about 2-3 hours.  

Preheat the oven to 350.  Line 2 baking sheets with parchment paper.  Spray the parchment paper with cooking spray - this helps to make sure the cookies won't stick.  Scoop out a tablespoon of dough and put it onto the baking sheet.  Lightly wet your fingers and form the dough into a circle, and lightly press down on the top of the cookie to flatten it slightly.  Bake the cookies for 13 minutes.  

Remove from the oven and cool on the pan for 5 minutes.  Then transfer to a cooling rack to cool completely.  The cookies will store in an airtight container for days.