In a large bowl, cream together the butter and cream cheese. Blend in the egg yolk and vanilla. Gradually beat in the cake mix. Dough will be stiff. Cover, and chill for 30 minutes.
Preheat oven to 375 degrees F (190 degrees C). Grease cookie sheets. Drop dough by rounded teaspoonfuls onto the prepared cookie sheets.
Bake for 8 to 10 minutes in the preheated oven, or until firm. Cool on wire racks.
