Cream the Butter and Sugars: In a large bowl, cream together the softened butter, granulated sugar, and powdered sugar until light and fluffy (3-5 minutes).
Add the Egg Yolk and Vanilla: Beat in the egg yolk and vanilla extract until well combined.
Combine Dry Ingredients: In a separate bowl, whisk together the all-purpose flour, cornstarch, and salt.
Gradually Add Dry Ingredients to Wet Ingredients: Slowly add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix the dough.
Chill the Dough: Wrap the dough in plastic wrap and chill in the refrigerator for at least 30 minutes.
Shape the Cookies: Preheat oven to 350°F (175°C). Roll the dough into 1-inch balls. Place the balls on a baking sheet lined with parchment paper.
Make the Thumbprints: Use your thumb (or the back of a small spoon) to make an indentation in the center of each cookie.
Fill with Jam: Fill each indentation with about ¼ teaspoon of your favorite jam.
Bake the Cookies: Bake for 10-12 minutes, or until the edges are lightly golden brown.
Cool and Enjoy: Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
