Start by beating the soft butter with sugar in a mixing bowl for about 3-4 minutes, until the mixture turns a lovely pale white.
Next, beat in the egg and pour in the vanilla extract. Keep mixing for another couple of minutes until everything is well combined and creamy.
Gradually add the flour mixed with cornstarch and salt in small portions. The dough should be soft and slightly sticky. If it’s too dry, don’t hesitate to add a tiny bit of water.
Wrap the dough in cling film and pop it into the refrigerator for about 30 minutes to an hour.
Preheat your oven to 350°F (175°C). While the oven heats up, form small balls of dough, no larger than 1 inch in diameter. Roll each ball in sugar and place them on a parchment-lined baking sheet.
Use your thumb or the back of a small spoon to make a heart-shaped print in the center of each ball. Then, fill that little heart with your choice of jam.
Bake the cookies for about 10-12 minutes, until they’re lightly golden.
Once baked, let the cookies cool on the baking sheet for a few minutes. Feel free to add a little more jam while the cookies are still warm.