Preheat and Prepare: Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper or silicone mats.
Prepare Dry Ingredients: Whisk together the flour, cornstarch, and salt in a separate bowl. Set aside.
Cream Butter and Sugars: In a large bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy (about 2-3 minutes).
Add Wet Ingredients: Mix in the egg yolk and vanilla extract, scraping the bowl as needed.
Incorporate Dry Ingredients: Gradually add the dry ingredients, starting with a spatula to combine, then mixing gently with the hand mixer until just combined.
Shape the Cookies: Scoop 1-tablespoon portions of dough, roll into balls, and optionally coat in sugar.
Create the Thumbprints: Place the dough balls on the baking sheets, spacing them 2 inches apart. Press a thumb or spoon into the center of each ball to create an indentation.
Fill with Jam: Add about ½ teaspoon of your favorite jam or preserves to each indentation.
Bake: Bake for 12-15 minutes, or until the edges are golden, being careful not to overbake.
Cool and Enjoy: Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.