Preheat oven to 350 degrees F.
In a mixing bowl combine butter and sugar.
Beat until creamy.
Add in vanilla and egg and mix until well blended.
In a separate bowl combine flour, cornstarch, and salt.
Slowly add flour mixture in batches into butter mixture.
Mix until well incorporated.
Place cookie dough info the refrigerator and cool overnight.
When ready to bake take 1 tablespoon of dough and form into a ball.
Place dough on a cookie sheet.
Make a small well in the center of each cookie using either your thumb or the back of a 1 teaspoon measuring spoon. (you may need to redo the well after baking.
Place cookies in the oven and bake for 12-15 minutes or until edges are brown.
Remove cookie from oven and let cool for about a minute on baking sheet before transferring to cooling rack.
Place a glass bowl over a pot of boiling water.
Melt chocolate with butter and corn syrup.
Remove from heat and let cool for about 10 minutes.
Place cooled chocolate into well of each cookie (about 1 teaspoon) and let set.