Cream together the butter and both the brown and white sugar.
Add the egg yolk and vanilla extract.
Combine the flour, cornstarch and salt together in a bowl and then add it to the wet ingredients.
Mix until it forms a soft dough.
Divide the dough into 24 pieces and roll each piece into a ball.
Using the bottom of a fork, press diagonally into one end of the cookie ball and then rotate the fork and press again until you have made a heart shape.
Place the cookies on a baking tray lined with parchment paper and place the tray in the refrigerator for 30 minutes.
While the cookies chill, add the jam to a microwave-safe bowl. Heat it up in the microwave for about 10 seconds. You just want to warm the jam up a little so it's easier to add to the unbaked cookies.
Remove the cookies from the refrigerator and fill the imprints with the jam.
Bake the cookies in a preheated 375°F oven for 10 to 12 minutes or until the edges are slightly golden brown.
Remove the cookies from the oven and transfer to a cooling rack.