Cream Together: In a stand mixer, cream together the softened unsalted butter, sugar, and light brown sugar until light and fluffy, about 3-5 minutes.
Add Egg and Vanilla: Next, add the egg yolk and vanilla extract. Beat until well combined.
Whisk Dry Ingredients: In a separate bowl, whisk together the all-purpose flour, cornstarch, and salt.
Combine Wet and Dry: Gradually add the dry mixture to the wet ingredients on low speed until just combined.
Roll Dough Balls: Roll the dough into 1 tablespoon-sized balls and place them on a wax-paper-covered plate or small cookie sheet.
Sugar Coating: If desired, roll each ball in granulated sugar.
Create Indentations: Press your thumb gently into the center of each cookie dough ball to make a thumbprint.
Chill the Dough: Transfer the cookie dough to the freezer and chill for 30 minutes.
Preheat Oven: Preheat your oven to 375°F (190°C).
Heat the Jam: Heat the jam in the microwave for 5-10 seconds.
Fill Thumbprints: Spoon the warmed jam into each thumbprint.
Bake the Cookies: Bake at 375°F (190°C) for about 11 minutes.
Cool Down: Allow the cookies to cool completely on the baking sheet before serving.
