Cream the butter and sugar together in a bowl until light and fluffy.

Add the egg yolk to the creamed mixture and mix well.

Sift the flour and salt together in a separate bowl.

Gradually add the dry ingredients to the wet ingredients and mix until just combined.

Roll the dough into small balls, approximately 1 inch in diameter.

Dip each ball in the unbeaten egg white.

Roll the egg-coated ball in finely chopped nuts, ensuring it is evenly coated.

Place the nut-covered balls on a buttered baking sheet, leaving some space between each cookie.

Use a thimble to gently press down the center of each cookie to create a small indentation.

Bake in a preheated 325°F (160°C) oven for 5 minutes.

Remove from the oven and press down the center of each cookie again with the thimble.

Return the cookies to the oven and bake for an additional 10 to 15 minutes, or until lightly golden.

Remove the cookies from the oven and let them cool slightly on the baking sheet.

While the cookies are still warm, fill the indentation in the center of each cookie with preserves or melted chocolate.