With an electric mixer, beat together margarine, powdered sugar, and salt until soft and fluffy.  Beat in vanilla extract.  Add the flour, 1/2 cup at a time, and beat until the mixture is well blended.  Cover and chill for an hour or until firm.  Roll dough into 1" balls (makes about 4 dozen) and place on a baking stone or cookie sheet.  Bake at 350 for 10-12 minutes (cookie will not brown but be barely firm and seem a little underdone).  Transfer to a cooling rack and let cool.  To make filling: microwave chocolate chips and margarine until melted.  Stir until smooth.  Using a butter knife, spread one cookie with a little bit of chocolate filling; attach a second cookie to the chocolate to create a sandwich.  Allow to cool.