Rich, tender and buttery cookies filled with sweet fruit jam. A traditional holiday treat with tempting flavor. There are also many different ways to adapt them too (see notes).
Pour sugar and salt into bowl of stand mixer and whisk.
Add butter to sugar and blend mixture using the paddle attachment until well creamed together. Scrape down bowl.
Mix in egg yolks and vanilla extract, scrape down bowl.
Add flour and mix until combined.
Scoop dough out and shape into 1-inch balls (just slightly over 1 Tbsp each, or 20g). Place on a flat dish lined with parchment paper and make an indent in each cookie using thumb or a wine cork (just don't push all the way through the dough, but make large enough to fit 1/2 tsp of jam).
Freeze the shaped cookie doughs for 20 minutes. Meanwhile preheat oven to 350 degrees.
Remove half of the shaped thumbprints and place on a parchment paper lined baking sheet spacing 2-inches apart. Fill with 1/2 tsp jam in each indent.
Bake in preheated oven 13 to 15 minutes until nearly set. Let cool several minutes then transfer to a wire rack to cool completely.
Repeat with remaining cookie dough. Store cookies in an airtight container at room temperature.
Notes
Coat cookies in nuts or coconut: Save the egg whites. Whisk the egg whites until frothy then brush egg white over cookie dough balls. Then roll the cookie dough balls in finely chopped nuts or shredded sweetened coconut before making indentation in center.
Add other fillings: Cookies can also be baked unfilled then after baking gently re-press the indentation back down with a teaspoon, fill with dulce de leche, melted caramel, lemon curd, frosting, or chocolate ganache.