1. Make the Cookie Dough:

1. In a large bowl, cream together the butter and sugar until light and fluffy.
2. Add the egg yolk and vanilla extract, mixing until well combined.
3. In a separate bowl, whisk together the flour and salt. Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.

2. Chill the Dough:

1. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes, or until firm.

3. Shape the Cookies:

1. Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
2. Roll the dough into 1-inch balls. If desired, roll the balls in finely chopped nuts for added texture.
3. Place the balls on the prepared baking sheets, spacing them about 2 inches apart.

4. Create the Indentations:

1. Use your thumb or the back of a small spoon to make a shallow indentation in the center of each ball.
2. Fill each indentation with about 1/2 teaspoon of jam, being careful not to overfill.

5. Bake the Cookies:

1. Bake in the preheated oven for 12-15 minutes, or until the edges are lightly golden.
2. Allow the cookies to cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely.

6. Serve or Store:

1. Dust with powdered sugar before serving, if desired.
2. Store in an airtight container at room temperature for up to 5 days.