Preheat oven to 375℉

In a mixing bowl sift together the flour, cornstarch, and salt. Set bowl aside.

Into a large mixing bowl or stand mixer bowl, add the butter and mix until creamy.

Add in the white and brown sugar, and beat until all the ingredients are combined.

Add the egg yolk and vanilla extract in beat until combined.

Slowly add the flour mixture to the creamed butter sugar mixture. The dough will look a bit crumbly so scrap the sides and bottom of the bowl down occasionally. Continue to add the flour mixer in until all are combined. To finish mixing the dough, you can use your hands if your mixer strains.

Using a 1 tablespoon size cookie scoop, scoop the dough and form a ball by rolling the dough in between your hands.

Place dough balls onto a cookie sheet pan. Place into the preheated 375℉ oven and bake for 5 minutes.

Remove the cookies from the oven after 5 minutes and carefully press thumb into cookies. You could also press the back of the ½ teaspoon into the cookies.

Add ¼ teaspoon of jam into each cookie and bake for another 12-15 minutes or until edges are golden brown.

Place on a wire cooling rack