Cream together butter and sugar, then beat in egg yolks and vanilla extract. Whisk flour and salt separately, then gradually add dry ingredients to wet, mixing until just combined to form a soft dough.

Wrap the dough in plastic wrap and refrigerate for at least 30 minutes to make it easier to handle and prevent spreading.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. Roll dough into 1-inch balls and place them 2 inches apart on the prepared sheets.

Gently press an indentation into the center of each cookie ball using your thumb or a small spoon, being careful not to press all the way through.

Bake for 10-12 minutes until edges are lightly golden; the centers will set as they cool. Remove from oven, re-press indentations if needed, and cool on baking sheets for 5 minutes before transferring to a wire rack.

Place chopped chocolate in a heatproof bowl. Heat heavy cream until simmering, then pour over chocolate and let sit for 5 minutes before whisking until smooth and glossy.

Gently melt chocolate chips or chopped chocolate in a microwave or double boiler until smooth. Optionally, stir in a small amount of butter or powdered sugar.

Once cookies are completely cool, spoon or pipe the chocolate filling into the thumbprint indentations.

Allow the chocolate to set at room temperature or in the refrigerator for 20-30 minutes before serving.