With the rack in the middle position, preheat the oven to 375°F (190°C).
Butter a small 13 x 9 - inch (33x23 cm) sheet pan and line with parchment paper, letting it hangover two sides.
In a bowl, combine the flour, baking soda and salt.
In another bowl, cream the butter and brown sugar with a wooden spoon.
Add the egg and egg yolk.
Mix until smooth.
Add the dry ingredients and chocolate.
Using a spatula, spread the batter out evenly on the sheet pan.
Bake for 20 minutes or until the cookie is slightly golden but still soft in the centre.
Let cool completely. Cut into squares.
The cookie will keep for 3 days in an airtight container at room temperature.
NOTE: This batter also works well to make individual cookies. Form eight balls, using 2 tbsp of batter for each one, and place on a sheet pan lined with a silicone mat or parchment paper. Bake at 375°F (190°C) for 7 to 9 minutes or until the cookies are slightly golden but still soft in the center. Let cool on the sheet pan.
