Cream butter, brown sugar, and 1/3 cup of sugar together with an electric hand mixer in a large bowl.
Add egg yolk and vanilla extract and beat until blended.
In a separate bowl, whisk together flour, cornstarch, and salt.
Gradually add dry ingredients to wet ingredients beating until combined (dough will be pretty dry).
Chill cookie dough in the freezer for 20 minutes.
Put remaining sugar in a shallow dish.
Form cookie dough into tablespoon-sized balls and roll until round.
Roll in a dish of sugar and place on cookie sheet.
Use the back of a teaspoon or thumb to make an indent in the center of the cookie. Preheat oven to 375.
Put preserves in a microwave-safe bowl and heat for 10-15 seconds until easy to stir.
Spoon jam into indentation until filled.
Bake for 11 minutes until edges are just beginning to turn golden brown.
Allow to cool completely on the baking sheet.