In medium bowl, whisk together flour, cornstarch and salt until well combined.

In bowl of stand mixer fitted with paddle attachment, beat butter at medium speed for 30 seconds. Add sugar and beat at medium speed for 2 to 3 minutes or until mixture is light and fluffy, scraping down sides of bowl occasionally.

Add egg yolks and vanilla and mix at medium speed until well combined.

With mixer on low speed, gradually add flour mixture until all of flour mixture is incorporated, scraping down sides of bowl as needed.

Cover mixer bowl with plastic wrap and transfer to refrigerator to chill for at least 1 hour.

Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper.

Using a medium cookie scoop or slightly heaping tablespoon, scoop dough and roll into 24 balls in the palms of your hands. Flatten balls slightly and place evenly spaced about 1 inch apart, 12 balls on each cookie sheet.

Using the end of a small spoon or your thumb, gently press a heart shape into each ball. Fill each heart cavity with 1/2 teaspoon jam or preserves (you may need slightly less, even. Do not overfill).

Transfer cookies to freezer to chill for 10 minutes (if cookie sheets won't fit in your freezer, you can carefully transfer cookies to parchment-lined plates, then transfer them back to cookie sheets before baking).

Transfer cookies to oven and bake 12 to 14 minutes or until bottoms are golden brown. Transfer baking sheets to cooling racks and let cookies cool on pans for 5 minutes before transferring cookies to cooling racks to cool completely.