Cream the softened unsalted butter until smooth. Gradually add granulated sugar and brown sugar, beating until light and fluffy.

Mix in the egg yolk and vanilla extract until fully combined.

In a separate bowl, whisk together all-purpose flour, cornstarch, and salt. Gradually add to the butter mixture and stir until crumbly.

Chill the dough in the refrigerator for about 30 minutes.

Scoop tablespoon-sized portions and roll into balls. Optional: roll in granulated sugar.

Make indentations in the center of each cookie ball using your thumb.

Chill the cookies again for 30 minutes to maintain shape during baking.

Preheat the oven to 375°F (190°C). Fill indentations with raspberry preserves.

Bake for 11 minutes or until edges are lightly golden. Cool on the baking sheet for 10 minutes.

Transfer the cookies to a wire rack to cool completely.