Preheat the oven to 320°f / 160°c. Line two sheet pans with parchment paper.

In the bowl of a stand mixer fitted with with the paddle attachment, or in a medium bowl using a handheld electric mixer, cream together the powdered sugar, butter, and vanilla until light and creamy, 2-3 minutes.

Sift together the flour, corn starch and salt if using and add to the bowl, and mix until just incorporated.

Turn out the dough onto a work surface, press the dough together, and shape into a rectangle log about 3 ½ cm x 6 cm (1.4"x2.3"), squaring off the edges with a bench scraper.

Using a sharp knife, cut the shortbread into biscuits 1cm (0.4") thick, and arrange on the trays, leaving a little room for spreading.

Bake the shortbread for 16 to 17 minutes, until set and just beginning to turn golden around the edges. Remove from the oven and leave to cool on the pans, then transfer to a wire rack.

Store shortbread in an airtight container at room temperature for up to two weeks - best eaten the week that they are made.