Preheat oven to 350° and line 2 cookie sheets with parchment paper.
In a large bowl, combine butter and sugar until smooth. Add in the egg and mix well. Add in the flour, baking soda, cornstarch, and salt. Mix until well combined.
Scoop into balls using 1/3 cup measure. It should equal 6 even balls of dough. Place 3 dough balls on a cookie sheet, with enough room for them to spread. Slightly press down and sprinkle sugar on top.
Bake 14-15 minutes or until lightly browned around the edges.
Notes
Cookies will be very soft when they come out of the oven, but will set up as they cool.