Thumbprint Cookies

Ingredients

• 1 cup butter or margarine
• 1/2 cup brown sugar, firmly packed
• 2 large egg yolks
• 1 teaspoon vanilla extract
• 2 cups flour
• 1/4 teaspoon salt
• 2 large egg whites, slightly beaten
• 2-1/2 cups walnuts, finely chopped
• 1 cup fruit preserves