Pre-heat oven to 350°F.
Cream butter and sugar. Beat in egg yolk.
Sift flour and salt, then blend into butter mixture. Add vanilla extract.
Chill dough well.
Shape into 1inch balls and place on ungreased cookie sheet. Make indentation in center with thumb; fill with preserves, jelly, pecan or walnut half, dried fruit or chocolate kiss.
Bake in oven for 15 minutes or until lightly browned.
Cool slightly, remove to rack to finish cooling.
These keep well in tight containers.