Preheat oven to 350 degrees Fahrenheit.
In the bowl of an electric mixer cream together the butter, sugar and salt for 2-3 minutes on medium speed.
Add the egg white and mix until combined.
Whisk together the flour, baking soda and cocoa and add to the butter mixture and beat on low until the flour mixture is just combined.
Divide the dough in half. Roll each half between 2 pieces of parchment paper to a thickness of 1/8”. Place both sheets of dough in the freezer until solid.
Remove one sheet of dough from the freezer, peel off the top sheet of parchment paper, turn over and remove the other piece of parchment.
Cut the dough into 1 ½” squares. Place the dough back in the freezer for 15-20 minutes.
Place the cookies on a parchment lined baking sheet and bake for 10-11 minutes. Remove from the oven and let the cookies cool for 10 minutes. Transfer the cookies to a wire rack and allow to cool completely.
The cookies can be made 2-3 days ahead and stored in an air tight container.