Preheat oven to 350 degrees F. Position the oven racks to divide the oven into 1/3's. Line two (2) Cookie Sheets with Parchment Paper or Silicone Baking Mats. Fit a Pastry Bag with a medium-sized open star tip.
In a small bowl, stir together the flour and cocoa; set aside.
In a large bowl, using a whisk, beat the butter until it is light and creamy. NOTE: For the recipe to be successful, the butter must be very soft.
Whisk in the sugar and salt, then stir in the egg whites. NOTE: Do not be concerned when the mixture separates, as it will come together when you add the dry ingredients.
Gradually add the flour/cocoa mixture and blend only until incorporated. NOTE: You do not want to work the mixture too much once the flour is added. A light touch is what will give these cookies their characteristic crumbliness.
It is easier to work with this dough in batches. Spoon about 1/3 of the dough into your Pastry Bag. Pipe the dough into "W" shaped cookies, each about 2-inches long and 1 1/4-inches wide, and place 1-inch apart onto the prepared baking sheets.
If desired, you may pipe the cookies in any shape that you desire. If you do not have a Pastry Bag, you can use a plastic bag. To use one, you just need to cut off the closed tip after filling the bag with dough to squeeze out an inch-wide portion of dough at a time.
Bake the cookies for 10 to 12 minutes (no more) or until they are set but neither browned nor hard. Remove from oven. Using a wide metal spatula, transfer the cookies to a wire cooling rack to cool to room temperature. Repeat with the remaining dough, making sure that you don't put the dough on hot baking sheets.
Before serving, you can dust the cookies with powdered sugar if you desire.
The cookies will keep well in a tightly covered tin at room temperature for approximately 1 week. They can be wrapped airtight and frozen for up to 1 month.
* Because the dough is made with powdered (confectioners) sugar, the cookies are softer and more tender than most butter cookies.
** Lightly beat 2 large egg whites, then measure out 3 tablespoons.
Viennese Chocolate Sables Cookie Recipe: https://whatscookingamerica.net/Cookie/ChocSableCookies.htm