Preheat oven to 325d F. In a large bowl cream butter and sugar using an electric mixer. Add vanilla, scraping down bowl as needed. Blend in flour and salt, mixing until thoroughly combined. Roll tablespoonfuls of dough into small balls about 3 cm in diameter. Place dough balls on lightly greased cookie sheets about 3 cm apart. Press down the center of each ball with a spoon, forming a depression. Fill each with a teaspoonful of preserves or nuts. Bake 15-20 minutes or until golden brown. Transfer cookies immediately to a cool, flat surface. When cookies are completely cool, dust them lightly with confectioners' sugar.