Beat the butter for 30 seconds.

Add the sugar, yolks, and vanilla. Beat until combined.

Beat in the flour (you may need to stir in the last bit of the flour)

Cover and chill the dough for about 1 hour.

Roll dough into 1-inch balls and place on greased cookie sheet.

Press your thumb into the center of the balls. (be sure to press well!)

Bake at 375 degrees for 10 minutes or until golden brown on the edges.

Cool and fill with jam.