Preheat oven to 350°F.
In a medium bowl, combine flour, cream of tartar and salt, and set aside.
In a mixing bowl, cream butter and sugar together until fluffy.
Add eggs, one at a time, beating well after each.
Stir in vanilla.
Add in flour mixture and blend well.
Roll dough into 1-inch balls and place on a cookie sheet.
Using a floured melon baller, or your thumb, make a “thumbprint” in the middle of each ball of dough.
Spoon jam into a plastic bag and snip off a corner in the bottom of the bag.
Pipe jam into the thumbprint cookies.
Bake for 11-13 minutes at 350°F.