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.
