Thumbprint Cookies

Name: Thumbprint Cookies
Prep time: 20 min
Cook time: 11 min
Total time: 31 min
Cuisine: American
Ingredients: 1 cup unsalted butter (softened to room temperature); ⅓ cup sugar; ⅓ cup light brown sugar, tightly packed; 1 large egg yolk; ¾ teaspoon vanilla extract; 2 ¼ cup all purpose flour; 2 teaspooons cornstarch; ½ teaspoon salt; ½ cup sugar (for rolling (optional)); ⅓ cup jam or preserves (flavor of your choice -- I used raspberry preserves for this recipe)