In a large mixing bowl, cream the butter, brown sugar, sugar together until nice and creamy. Add egg yolk and vanilla and mix until smooth. 

Gradually add the flour, cornstarch, and salt. The dough will seem dry and you may need to finish mixing the dough by hand so that all the ingredients are combined. 

Place remaining ½ cup of granulated sugar on a plate and set aside.

Using a 1 tablespoon cookie scoop, scoop balls of dough one at a time {finishing by rolling between the palms of your hands to create a nice round ball} and roll balls on the plate of granulated sugar making sure to coat. 

Place cookie dough balls on a baking sheet. 

Using the back of a rounded teaspoon, press it into the top of the dough to create a small well. 

Refrigerate cookies for 1 hour. 

Preheat the oven to 375 F. 

Once the cookies are chilled, spoon a small amount of jam {filling the well about ¾ full} with jam.

Line another cookie sheet with parchment paper and place cookies 2 inches apart on the cookie sheet and bake for 11 minutes {or until the edges have a hint of golden brown to them}. 

Allow the cookies to cool completely on the baking sheet before removing them. 

This recipe makes about 30 thumbprint cookies.