Prepare your oven and equipment. Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. This prevents sticking and helps the cookies bake evenly.
Cream butter and sugar until fluffy. In a large bowl, beat the softened butter and granulated sugar together for about 3 minutes until the mixture is light and creamy. This incorporates air for a tender texture.
Add flavor and egg yolks. Beat in the vanilla extract, almond extract (if using), and egg yolks until fully incorporated and smooth. The mixture should look slightly pale yellow and have a silky consistency.
Incorporate dry ingredients carefully. Add the flour and salt to the butter mixture, mixing at low speed just until combined. Overmixing will develop the gluten and result in tough cookies.
Shape the dough into balls. Roll the dough into 1-inch balls (about 1 tablespoon each). If using nuts, roll the balls in chopped nuts now.
Create the thumbprint indentation. Place the balls 2 inches apart on the prepared baking sheets. Using your thumb or the back of a 1/4 teaspoon measuring spoon, press gently into the center of each ball to create a well for the jam.
Fill with preserves. Spoon about 1/2 teaspoon of fruit preserves into each indentation, being careful not to overfill. The jam will spread slightly during baking.
Bake until just set. Bake for 10-12 minutes or until the edges are just barely turning golden. The cookies should not brown significantly—this ensures they stay soft.
Cool properly for best texture. Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. They’ll firm up slightly as they cool but remain tender.
Add final touches if desired. Once completely cool, dust lightly with powdered sugar for a snow-kissed appearance if desired.