Place one of your oven racks in the upper third of the oven (this will keep the cookies from getting too brown on the bottom). Preheat oven to 325˚F. Line two sheet pans with parchment baking paper.
In a medium size bowl, whisk the butter until very light and fluffy, about 20-30 seconds. Add the powdered sugar fand stir until completely incorporated, then add the cornstarch, vanilla and salt. Stir until nice and smooth.
Lastly, add the flour. Stir until all of the flour has been incorporated. At first, the dough will be very crumbly. Keep stirring, turning the bowl and scraping the sides and bottom as you go.
The dough will be soft but you should be able to scoop it up and roll it into a ball. If it seems too soft to roll add 1 tablespoon of flour and stir until incorporated. This can be repeated one more time. If the dough still seems too soft to roll, put the bowl in the refrigerator for 15 minutes, then proceed as directed below.
Using a small cookie scoop, make equal-sized scoops of dough. Roll each scoop into a ball then roll each ball in the sugar. (The sugar will make it a little easier to roll the dough into nice round balls.) Roll the ball again to a nice round shape.
Arrange the dough balls on the prepared sheet pans, spacing them about 2 inches apart. With a flat bottom glass, gently flatten each cookie slightly, to about ½ inch in height.
Bake for 18-22 minutes or until just beginning to turn light golden brown. Remove from oven and place on a cooking rack.