- In a bowl, combine the flour and cocoa powder. Set aside.
- In another bowl, cream the butter, sugar and salt with an electric mixer until smooth. Add the egg white and beat until smooth. At low speed or with a wooden spoon, stir in the dry ingredients and chocolate. Divide the dough in half and place each piece on a sheet of plastic wrap.
- With the plastic wrap, shape and roll each piece of dough into a 1 ½-inch (4 cm) diameter log. Twist the plastic wrap at each end of the logs to seal. Refrigerate for 2 hours or until the dough is very firm to the touch (or freeze at this stage).
- With the rack in the middle position, preheat the oven to 350°F (180°C). Line two baking sheets with parchment paper.
- On a work surface, unwrap the logs. Cut into ¼-inch (7 mm) slices. Place the cookies on the baking sheets.
- Bake, one sheet at a time, for about 12 minutes or until the edges of the cookies are firm but still soft in the centre. Let cool completely on the sheets.
- These shortbreads will keep for about two weeks in an airtight container in a cool, dry place.