In a mixing bowl, cream the butter, icing sugar, and vanilla together until pale and creamy (a couple of minutes).
Add the egg yolk and beat until just combined.
Gradually add the flour and salt (if using) and beat until the mixture forms a crumbly dough. It may seem dry at first, but keep mixing and it will come together.
Use your hands to form the dough into a disc. Wrap tightly with cling film, and chill for 30-60 minutes. The dough can stay in the fridge like this for up to 3 days, if you'd like to make it in advance.
Remove the dough from the fridge and roll it out between two pieces of parchment paper until roughly 6-7 mm (¼ inch) thick. Cut the dough into 6 x 7 cm rectangles using a ruler as a guide. Then, cut each rectangle diagonally to form 2 triangular "bread slices." Gather the dough scraps, re-roll, and cut more cookies. If the dough starts to feel quite soft or greasy at any point, place it back in the fridge for 5-10 minutes to firm up.
Place the formed shortbread on a parchment-lined baking sheet, making sure there is at least 2 cm of space around each cookie. Chill for 15-30 minutes to minimise spreading. In the meantime, preheat your oven to 160℃ (140℃ fan/325℉).
Bake the shortbread for 12-15 minutes, or until lightly golden around the edges. If they have spread during baking, you can use a bench scraper or any tool with a straight edge to carefully straighten the sides while warm. Allow to cool and firm up on the baking sheet for a few minutes before transferring to a wire rack.