Christmas Tree Sandwiches

For the dough:
1 cup unsalted butter, softened , 2 cups flour
1/4 teaspoon salt , 1/3 cup milk
3/4 cup sugar

for the filling:
1/4 cup Crisco shortening , 1/4 cup unsalted butter, softened
1/2 teaspoon vanilla extract , 2 cups powdered sugar
1 1/2 tablespoons milk
green food coloring , 1 tube brown decorating icing, with writing tip

In large bowl with electric mixer, beat butter until creamy, about 1 minute. With mixer on low, alternately add flour, salt, and milk and beat until well blended. Wrap dough in plastic wrap and refrigerate until firm, about 1 1/2 hours. Preheat oven to 375F. 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" cookie cutter. Press cookies into sugar to coat on both sides then place on ungreased cookie sheets. Prick each cookie all over with a fork, for decoration. Bake cookies 9 to 11 minutes until very lightly browned. Cool on sheets 2 minutes then transfer to wire racks to cool completely. While cookies are baking, prepare the filling. In a large bowl, beat shortening and butter until creamy with an electric mixer. Beat in vanilla. Gradually add powdered sugar and milk. Add food coloring to tint the filling pale green. 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.