Preheat oven to 350F
In KitchenAid cream together butter, sugars and vanilla.
Stir in eggs, one at a time.
In separate bowl whisk together flour, salt, cornstarch and baking soda.
With mixer on low-medium speed, gradually add flour mixture to butter/sugar mixture, pausing periodically to scrape down sides of bowl with spatula.
Drop cookie dough by 1 1/2" balls into cups of two mini-muffin tins and bake on 350F for 10-12 minutes (until slightly golden-brown around edges).
Remove cookies from oven and allow to cool for 1 minute before creating a crater in the center of each cup by pressing hole halfway into cup using rounded teaspoon (or similar utensil).
Allow cookies to cool completely in muffin tins before filling.