(1). In a mixing bowl, beat butter on low until creamy, alternately add flour, salt, and milk and beat until well blended.You may wish to add color to the cookies at this point or color the frosting.
(2). Wrap dough in plastic wrap and refrigerate until firm, about 1 1/2 hours.
(3). Preheat oven to 375F.
(4). Spread sugar on a plate. On a floured surface, roll one third of the dough to 1/8″ thick. Cut out tree shapes with a 2″ – 4″ cookie cutter. Press cookies into sugar to coat on both sides then place on ungreased cookie sheets.
(5). Bake cookies 9 to 11 minutes until very lightly browned. Cool on sheets 2 minutes then transfer to wire racks to cool completely.
(6). While cookies are baking, prepare the filling. In mixing bowl, beat shortening and butter until creamy. Beat in vanilla. Gradually add powdered sugar and milk. Add food coloring if desired. Beat on high until the filling is light and fluffy; appx. 3 minutes. Spread filling on half of the cookies and sandwich with the remaining halves.