In the bowl of a stand mixer, cream the butter, cream cheese and sugar until light and fluffy, 5-7 minutes.
Beat in egg yolk and vanilla. Add flour and mix well. Cover and refrigerate for 1 hour or until easy to handle.
Preheat oven to 350° F.
Shape dough into 1-in. balls. Place 2 in. apart on parchment covered cookie sheets. Using the end of a wooden spoon handle or bottom of a measuring spoon, make a ½-in.-deep indentation in the center of each ball; fill each with guava marmalade.
Bake for 10-12 minutes or until set. Remove to wire racks to cool.