Sure, strawberry jam is our favorite, but any jam you like is fine (bought or homemade jam). We love to make classic thumbprint cookies with seedless raspberry jam or red currant jam. What you must consider is that the jam should be perfectly smooth (without any seeds or tiny fruit pieces) and that it should be a tart sort. Sugar-reduced jam is my favorite choice; the contrast between the sweet dough and the tart jam makes these thumbprints so great.