Preheat oven to 375 degrees. In large mixing bowl with electric mixer combine butter, shortening, sugar, egg yolk, vanilla and salt, beating until light and fluffy. Blend in flour gradually on low speed. In small mixing bowl, slightly beat egg white. Form dough into 1 inch balls; dip into egg white. Roll each ball in sugar or chopped nuts. Place on ungreased baking sheet. Using thumb, form a depression in center of each cookie. Fill with 1/2 teaspoon of jelly or jam. Bake cookies 12 to 15 minutes until set and very light brown. Cool on wire rack. Store cookies tightly covered.

Makes about 2 dozen cookies

Cooks tip

All ovens vary in temperature and I have found that if you bake the UNFILLED cookies with depression for 5 minutes then remove from oven, fill with jam or jelly and bake for an additional 7 minutes.