In a large bowl, cream together the butter, cream cheese and sugar untill light
and fluffy. Beat in egg yolk and vanilla extract. Add flour and mix well.
and refrigerate for 1 hour.
Roll dough into 1-inch balls. Place 2 inches apart on an ungreased cookie sheet.
Using the end of a wooden spoon handle, make a 1/2-inch deep indention in the center
of each dough ball; fill with about 1/4 teaspoon jam.
Bake in preheated 350 degree F oven for 10-12 minutes or until set.