Leave the butter at room temperature before starting to bake. Mix the butter, powdered sugar, cornstarch, and salt until smooth and silky.
1 cup Butter, ¾ cup Powdered sugar, 1 tablespoon Cornstarch
Add the egg yolk and vanilla extract.
1 Egg yolk, 1 teaspoon Vanilla extract
Add the flour in two or three passes and mix well after each. You would have a soft and smooth dough in the end.
2¼ cup All-purpose flour, ¼ teaspoon Salt
Shape the dough into two discs, cling wrap, and chill for one hour.
Lightly dust a clean surface with powdered sugar and roll out the dough. You can place the dough between two pieces of wax paper to make rolling easier. Once the dough is ¼ of an inch thick, use the round cookie cutter to cut the dough.
Use the end of a teaspoon to make two indentations that cross each other at the end. We want to make a heart-shaped indent. Place the raw cookies on a lined baking sheet in the fridge for 15 to 20 minutes.
In the meantime, preheat the oven to 325F. Bake the cookies for 8-10 minutes or until the edges look set. Do not overbake them, as it makes them dry and brown.
Let them cool down for a few minutes, then transfer them to the cooling rack to cool down fully.
Place about one teaspoon of the jam filling into the heart indentation.
⅓ cup Raspberry jam