Pre-heat oven to 350ºF (175ºC). Mix butter, shortening sugar, egg-yolk and vanilla thoroughly. Work in flour and salt until dough holds together. Shape dough by teaspoonfuls into 1 in. balls. Beat egg-white slightly dip each ball into eggwhite; roll in the finely chopped nuts.
Place 1 in. apart on an ungreased baking sheet; press thumb deeply into center of each. Bake for about 10 minutes or until light brown.
Remove immediately from the baking sheet and cool on a wire rack. Fill thumb prints with jelly or frosting.