1. In a medium mixing bowl, whisk together the flour, cornstarch, and salt. Set aside.

2. In a separate large bowl, use an electric mixer to beat the softened butter and sugar together until light and fluffy (about 2–3 minutes).

3. Mix in the egg yolks one at a time, followed by the vanilla extract, until well combined.

4. Gradually add the dry ingredients to the wet ingredients, mixing just until a soft dough forms.

5. Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

6. Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

7. Roll the chilled dough into small balls (about 1 tablespoon each) and gently flatten them before cutting with a heart-shaped cookie cutter.

8. Using your thumb or the back of a spoon, press a small heart-shaped indentation into the center of each cookie.

9. Spoon a small amount of strawberry or raspberry jam into the indentation of each cookie, being careful not to overfill.

10. Place the cookies on the prepared baking sheets, spacing them about 1 inch apart. Bake for 12–15 minutes, or until the edges are lightly golden.

11. Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.