- With the rack in the middle position, preheat the oven to 350°F (180°C). Line two baking sheets with silicone mats or parchment paper.
- In a bowl, using a wooden spoon, combine the butter with ½ cup (105 g) of the sugar. Add the egg yolk and vanilla. Mix until smooth. Add the flour, cocoa powder and salt.
- Using a 1-tbsp (15 ml) ice cream scoop, form the mixture into balls. Finish rolling the cookies with your hands, then roll in the remaining sugar. Place on the baking sheets, evenly spacing them out.
- Bake one sheet at a time for 14 minutes (see note). Let cool completely on the baking sheets.
The surface of the cookies will crack slightly but resist the urge to remove them from the oven before the indicated cooking time. If you do take them out too early, they will still be delicious but will not have the classic shortbread cookie texture.