In a stand mixer, cream the softened unsalted butter until smooth and fluffy, about 2-3 minutes on medium speed. Gradually add in the granulated sugar and brown sugar, beating until light and airy, about 2-3 minutes.

Add the egg yolk and vanilla extract to the creamed mixture. Beat on medium speed until fully incorporated, about 1 minute.

In a separate bowl, whisk together the all-purpose flour, cornstarch, and salt. Gradually add the dry mixture to the wet ingredients, mixing on low speed until the dough clumps together, about 1-2 minutes.

Scoop out portions of dough and roll into balls, about 1 inch in diameter. Place on a lined baking sheet and press a well into the center of each ball.

Chill in the freezer for 30 minutes to maintain shape while baking.

Preheat the oven to 375°F (190°C).

Warm raspberry preserves in a microwave-safe bowl for about 20 seconds until slightly soft.

Remove chilled cookies from the freezer and fill each thumbprint with the warmed raspberry preserves. Bake for approximately 11 minutes until edges are set and lightly golden.

Allow cookies to cool on the baking sheet for about 10 minutes before transferring to a wire rack.