In a medium bowl, combine together flour, salt, and cornstarch. Set aside.
Line a baking tray with parchment paper and set aside.
In a large bowl, add softened butter and beat it with a hand mixer or with a rubber spatula for 1 minute.
Next, add sugar to the butter. Cream together butter and sugar for 3 minutes with your hand mixer. Scrape sides and mix it again for a minute until light and fluffy.
Add some vanilla extract and mix it well.
Transfer the dry ingredients slowly to the sugar and butter mixture until combined. Scrape the sides of the bowl and mix again. I like to use my hands to combine and make the dough.
Scoop the cookie dough into a 1 tbsp sized ball and roll it into a smooth ball. Transfer the cookie to a baking tray.
Make an indentation in the center of a cookie by pressing it gently with your thumb or back of a rounded teaspoon. Repeat until all the dough has been used. Notes: Some cracks are normal but you can gently smooth them with your fingers or leave them there. Place them in the prepared baking sheet 2 inches apart.
Cover the baking tray with a kitchen towel. Chill the cookies in the refrigerator for 30 minutes.
Place the jam/preserve in a microwave safe bowl, heat it for 5-6 seconds, stir it with a spoon.
Preheat the oven to 350℉/180℃
Fill each indentation with about ¼ tsp of jam.
Bake the cookies for about 13-15 minutes, or until the edges are slightly brown. Remove them from the oven and let them sit on a baking tray for 5 minutes. Transfer them to a cooling rack and enjoy once they cool down completely.