Preheat oven to 350ºF. Lightly spray a 12 hole muffin pan with cooking spray and set aside.
Pull out and measure all of the ingredients.
Cream butter and sugar together until creamy. Beat in the egg and vanilla until well blended.
In a separate bowl, add the flour, salt, baking soda, and cornstarch. Mix together.
Beat the dry ingredients into the wet ingredients until well combined.
Dough will be very thick.
Each ball should be about a ¼ cup of dough.
In each muffin tin, press cookie dough in bottom and about half way up the sides, forming a cup shape.
Bake for 20-22 minutes or until cups lightly brown.
Remove from oven and let cool about 5 minutes. Then transfer to a cooling rack. If the centers have puffed up too much, then use the end of a wooden spoon, glass, or the bottom of a spoon to press the center down. Allow to cool completely before filling.