Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone baking mats.
In a large mixing bowl, cream the softened unsalted butter and granulated sugar together using an electric mixer on medium-high speed for 2 to 3 minutes until the mixture is light and fluffy.
Add the egg yolks and non-alcoholic vanilla extract to the butter mixture and beat until well combined, scraping down the sides of the bowl as needed.
In a separate bowl, whisk together the all-purpose flour and fine sea salt.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed just until a soft dough forms and no streaks of flour remain.
Scoop the dough into 1-inch balls (about 1 tablespoon each) and roll them between your palms until smooth.
Place the balls 2 inches apart on the prepared baking sheets. Use your thumb or the back of a rounded 1/2 teaspoon measuring spoon to press a deep indentation into the center of each cookie.
If the edges crack significantly, use slightly damp fingers to gently press the dough back together for a smooth finish.
Chill the baking sheets with the shaped dough in the refrigerator for at least 30 minutes; this prevents the cookies from spreading and losing the thumbprint shape during baking.
Remove from the refrigerator and spoon approximately 1/2 teaspoon of fruit preserves into the center of each indentation.
Bake for 10 to 12 minutes, or until the edges of the cookies are set and just barely beginning to turn a light golden brown.
Allow the cookies to cool on the baking sheet for 5 to 10 minutes before transferring them to a wire rack to cool completely.