First line a baking sheet with parchment paper or silpat.
Start by beating together the butter, both sugars, and salt until light and creamy. Add in the egg yolk ( or small egg) and vanilla and beat for another minute or so.
Add in the flour and corn starch and fold using a spatula.
When the dough forms, divide it into 18 equal golf sized balls and place them on the prepared baking sheet. I used a small cookie scoop to do this. Slightly flatten the balls and stamp with the a flour dusted stamp until it is about 1cm thick. At this point you can leave the cookies as they are or get a circle cookie cuter and cut the outside or the cookie to get more of a clean finish.
Place the baking sheet in the freezer for 30 minutes. About 20 minutes in, start preheating the oven to 400F.
After the stamped cookies have been chilling for 30 minutes. Pull them out and place them in the oven for 11 minutes, making sure to rotate the baking sheet halfway through. ( Make sure that your baking sheet is cold before baking, it will help keep the cookies from spreading) The cookies will be lightly golden around the edges and that's how you'll know they are done.
Remove the cookies from the oven and let them cool on the baking sheet.
Store these cookies in an airtight container at room temperature for up to 5 days, or in the freezer for up to 3 months.