Take your butter out of the fridge to come to room temperature.
When ready to bake, preheat oven to 325F. (not fan assisted), with the rack in the middle of the oven. Ready a 9x13-inch baking sheet or pan. Metal baking pans are best, as they have sharper corners, though you can use glass if that's all you have. *If using a glass pan, reduce oven temperature to 300F.
In a medium bowl, whisk together the flour, cornstarch and salt (omit salt if using salted butter). Set aside.
In a large bowl with an electric mixer or the bowl of a stand mixer with a paddle attachment, cream the butter for a minute, then add the icing sugar and mix at medium speed, until smooth, 1-2 minutes. With the mixer on low, gradually add the flour/cornstarch mixture to the bowl and mix until well combined.
Spoon dough onto a 9x13-inch baking sheet or pan and press into an even layer. Get it as even as you can so it will cook evenly. Use a fork to poke liberally into the dough every couple of inches, being sure to poke right to the bottom of the pan.
Bake in preheated oven for 40-45 minutes, or until just starting to take on a golden colour. Remove from oven and immediately sprinkle the top of the cookies liberally with granulated white sugar. While still hot, slice the cookies into desired shape in the pan. For the classic stick shape, aim for about 3/4" wide by 3" long. Allow the cookies to completely cool in the pan before attempting to remove from the pan. If you try to remove the warm, they will probably break.
Remove cookies to a cooling rack (even though they are already cool). Allow to stand for about an hour to dry out a bit more, then store in an airtight container. Cookies will keep well for several weeks and will actually develop more flavour the longer they sit. You can also freeze these cookies.
Notes
Be sure to read the notes above the recipe card for more tips for making this recipe.